libdm: add statistics groups

libdm: add statistics groups

Add a grouping facility to the libdm-stats library that allows the
user to bind several regions together as a group. Groups may be
used to aggregate data from several regions for reporting, or to
select and sort among large sets of regions.

A textual descriptor ("group tag") is associated with each group
and is stored in the first group member's aux_data field. The
tag contains the group member list and an optional alias for the
group, allowing the user to assign meaningful names to groups of
regions.

These descriptors are parsed in @stats_list message responses and
populate the resulting region and area tables with the group
structure.

Groups with overlapping regions are permitted but since this will
result in some events being counted more than once a warning is
printed in this case.

Nested and overlapping groups are not currently supported and
attempting to create these configurations results in error.