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/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