Add a new DynamicMeter class for runtime Meter extension

This commit is based on exploratory work by Sohaib Mohamed.
The end goal is two-fold - to support addition of Meters we
build via configuration files for both the PCP platform and
for scripts ( https://github.com/htop-dev/htop/issues/526 )

Here, we focus on generic code and the PCP support.  A new
class DynamicMeter is introduced - it uses the special case
'param' field handling that previously was used only by the
CPUMeter, such that every runtime-configured Meter is given
a unique identifier.  Unlike with the CPUMeter this is used
internally only.  When reading/writing to htoprc instead of
CPU(N) - where N is an integer param (CPU number) - we use
the string name for each meter.  For example, if we have a
configuration for a DynamicMeter for some Redis metrics, we
might read and write "Dynamic(redis)".  This identifier is
subsequently matched (back) up to the configuration file so
we're able to re-create arbitrary user configurations.

The PCP platform configuration file format is fairly simple.
We expand configs from several directories, including the
users homedir alongside htoprc (below htop/meters/) and also
/etc/pcp/htop/meters.  The format will be described via a
new pcp-htop(5) man page, but its basically ini-style and
each Meter has one or more metric expressions associated, as
well as specifications for labels, color and so on via a dot
separated notation for individual metrics within the Meter.

A few initial sample configuration files are provided below
./pcp/meters that give the general idea.  The PCP "derived"
metric specification - see pmRegisterDerived(3) - is used
as the syntax for specifying metrics in PCP DynamicMeters.

pcp/meters/entropy

@@ -0,0 +1,9 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[entropy]
+caption = Entropy
+avail.metric = kernel.all.entropy.avail / kernel.all.entropy.poolsize * 100
+avail.label = avail
+avail.suffix = %

pcp/meters/freespace

@@ -0,0 +1,11 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[freespace]
+caption = Freespace
+description = Filesystem space
+used.metric = sum(filesys.used)
+used.color = blue
+free.metric = sum(filesys.free)
+free.color = green

pcp/meters/ipc

@@ -0,0 +1,13 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[ipc]
+caption = SysV IPC
+description = SysV IPC counts
+msg.metric = ipc.msg.used_queues
+msg.color = blue
+sem.metric = ipc.sem.used_sem
+sem.color = green
+shm.metric = ipc.shm.used_ids
+shm.color = cyan

pcp/meters/locks

@@ -0,0 +1,15 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[locks]
+caption = File locks
+description = VFS file locks
+posix.metric = vfs.locks.posix.count
+posix.color = blue
+flock.metric = vfs.locks.flock.count
+flock.color = green
+readlock.metric = vfs.locks.posix.read + vfs.locks.flock.read
+readlock.color = red
+writelock.metric = vfs.locks.posix.write + vfs.locks.flock.write
+writelock.color = yellow

pcp/meters/memcache

@@ -0,0 +1,11 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[memcache]
+caption = Memcache
+description = Memcache Hits
+hit.metric = sum(memcache.hits)
+hit.color = green
+miss.metric = sum(memcache.misses)
+miss.color = blue

pcp/meters/mysql

@@ -0,0 +1,73 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[mysql_io]
+caption = MySQL I/O
+recv.metric = mysql.status.bytes_received
+recv.color = green
+recv.label = recv
+sent.metric = mysql.status.bytes_sent
+sent.color = blue
+sent.label = sent
+
+[mysql_keys]
+caption = MySQL keys
+description = MySQL key status
+key_blocks_used.metric = mysql.status.key_blocks_used
+key_blocks_used.label = color
+key_blocks_used.label = used
+key_reads.metric = mysql.status.key_reads
+key_reads.label = read
+key_reads.color = green
+key_writes.metric = mysql.status.key_writes
+key_writes.label = writ
+key_writes.color = blue
+key_read_requests.metric = mysql.status.key_read_requests
+key_read_requests.label = rreq
+key_read_requests.color = green
+key_write_requests.metric = mysql.status.key_write_requests
+key_write_requests.label = wreq
+key_write_requests.color = blue
+
+[innodb_buffer]
+caption = InnoDB pool
+description = InnoDB buffer pool
+created.metric = mysql.status.innodb_pages_created
+created.label = cr
+created.color = yellow
+read.metric = mysql.status.innodb_pages_read
+read.label = rd
+read.color = greed
+written.metric = mysql.status.innodb_pages_written
+written.label = wr
+written.color = red
+
+[innodb_io]
+caption = InnoDB I/O
+description = InnoDB I/O operations
+read.metric = mysql.status.innodb_data_read
+read.label = rd
+read.color = green
+written.metric = mysql.status.innodb_data.writes
+written.label = wr
+written.color = blue
+sync.metric = mysql.status.innodb_data_fsyncs
+sync.label = sync
+sync.color = cyan
+
+[innodb_ops]
+caption = InnoDB ops
+description = InnoDB operations
+inserted.metric = mysql.status.innodb_rows_inserted
+inserted.label = ins
+inserted.color = blue
+updated.metric = mysql.status.innodb_rows_updated
+updated.label = upd
+updated.color = cyan
+deleted.metric = mysql.status.innodb_rows_deleted
+deleted.label = del
+deleted.color = red
+read.metric = mysql.status.innodb_rows_read
+read.label = rd
+read.color = green

pcp/meters/postfix

@@ -0,0 +1,21 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[postfix]
+caption = Postfix
+incoming.metric = sum(postfix.queues.incoming)
+incoming.color = green
+incoming.label = in
+active.metric = sum(postfix.queues.active)
+active.color = blue
+active.label = act
+deferred.metric = sum(postfix.queues.deferred)
+deferred.color = cyan
+deferred.label = dfr
+bounce.metric = sum(postfix.queues.maildrop)
+bounce.color = red
+bounce.label = bnc
+hold.metric = sum(postfix.queues.hold)
+hold.color = yellow
+hold.label = hold

pcp/meters/redis

@@ -0,0 +1,41 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[redisxact]
+caption = Redis xact
+description = Redis transactions
+tps.metric = redis.instantaneous_ops_per_sec
+tps.color = green
+
+[redismem]
+caption = Redis mem
+description = Redis memory
+lua.metric = redis.used_memory_lua
+lua.color = magenta
+lua.label = lua:
+used.metric = redis.used_memory
+used.color = blue
+used.label = used:
+
+[redisclient]
+caption = Redis clients
+description = Redis clients
+type = bar
+blocked.metric = redis.blocked_clients
+blocked.color = blue
+blocked.label = blk
+clients.metric = redis.connected_clients
+clients.color = green
+clients.label = conn
+
+[redisconn]
+caption = Redis conn
+description = Redis connections
+type = bar
+reject.metric = redis.rejected_connections
+reject.color = magenta
+reject.label = fail/s
+total.metric = redis.total_connections_received
+total.color = blue
+total.label = conn/s

pcp/meters/tcp

@@ -0,0 +1,22 @@
+#
+# pcp-htop(1) configuration file - see pcp-htop(5)
+#
+
+[tcp]
+caption = TCP
+description = TCP sockets
+listen.metric = network.tcpconn.listen
+listen.color = green
+listen.label = lis
+active.metric = network.tcpconn.established
+active.color = blue
+active.label = act
+syn.metric = network.tcpconn.syn_sent + network.tcpconn.syn_recv + network.tcpconn.last_ack
+syn.color = cyan
+syn.label = syn
+wait.metric = network.tcpconn.time_wait
+wait.color = red
+wait.label = tim
+close.metric = network.tcpconn.fin_wait1 + network.tcpconn.fin_wait2 + network.tcpconn.close + network.tcpconn.close_wait + network.tcpconn.closing
+close.color = yellow
+close.label = clo