PCP: support for 'dynamic columns' added at runtime

Implements support for arbitrary Performance Co-Pilot
metrics with per-process instance domains to form new
htop columns.  The column-to-metric mappings are setup
using configuration files which will be documented via
man pages as part of a follow-up commit.

We provide an initial set of column configurations so
as to provide new capabilities to pcp-htop: including
configs for containers, open fd counts, scheduler run
queue time, tcp/udp bytes/calls sent/recv, delay acct,
virtual machine guests, detailed virtual memory, swap.

Note there is a change to the configuration file path
resolution algorithm introduced for 'dynamic meters'.
First, look in any custom PCP_HTOP_DIR location.  Then
iterate, in priority order, users home directory, then
local sysadmins files in /etc/pcp/htop, then readonly
configuration files below /usr/share/pcp/htop.  This
final location becomes the preferred place for our own
shipped meter and column files.

The Settings file (htoprc) writing code is updated to
not using the numeric identifier for dynamic columns.
The same strategy used for dynamic meters is used here
where we write Dynamic(name) so the name can be setup
once more at start.  Regular (static) columns writing
to htoprc - i.e. numerically indexed - is unchanged.

Process.h

@@ -56,6 +56,7 @@ typedef enum ProcessField_ {
    /* Platform specific fields, defined in ${platform}/ProcessField.h */
    PLATFORM_PROCESS_FIELDS
 
+   /* Do not add new fields after this entry (dynamic entries follow) */
    LAST_PROCESSFIELD
 } ProcessField;
 
@@ -267,7 +268,7 @@ typedef struct ProcessFieldData_ {
    /* Scan flag to enable scan-method otherwise not run */
    uint32_t flags;
 
-   /* Whether the values are process identifies; adjusts the width of title and values if true */
+   /* Whether the values are process identifiers; adjusts the width of title and values if true */
    bool pidColumn;
 
    /* Whether the column should be sorted in descending order by default */