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.

Makefile.am

@@ -39,6 +39,7 @@ myhtopsources = \
 	DateTimeMeter.c \
 	DiskIOMeter.c \
 	DisplayOptionsPanel.c \
+	DynamicMeter.c \
 	EnvScreen.c \
 	FunctionBar.c \
 	Hashtable.c \
@@ -93,6 +94,7 @@ myhtopheaders = \
 	DateTimeMeter.h \
 	DiskIOMeter.h \
 	DisplayOptionsPanel.h \
+	DynamicMeter.h \
 	EnvScreen.h \
 	FunctionBar.h \
 	Hashtable.h \
@@ -357,6 +359,7 @@ endif
 # --------------------------
 
 pcp_platform_headers = \
+	pcp/PCPDynamicMeter.h \
 	pcp/PCPProcess.h \
 	pcp/PCPProcessList.h \
 	pcp/Platform.h \
@@ -369,6 +372,7 @@ pcp_platform_headers = \
 	zfs/ZfsCompressedArcMeter.h
 
 pcp_platform_sources = \
+	pcp/PCPDynamicMeter.c \
 	pcp/PCPProcess.c \
 	pcp/PCPProcessList.c \
 	pcp/Platform.c \