libdm: add stats group and region iterators and properties

libdm: add stats group and region iterators and properties

Add support do dm_stats_walk*() to walk over the set of
available groups using the cursor embedded in the dm_stats
handle, and to obtain the type of the object at the current
stats cursor location. A set of flags is introduced to
control which objects are visited:

    DM_STATS_WALK_AREA
    DM_STATS_WALK_REGION
    DM_STATS_WALK_GROUP
    DM_STATS_WALK_ALL

A final flag suppresses visits to regions that contain only a
single area - since the aggregate of such a region is idential
to the area it contains this allows these duplicates to be
filtered out:

    DM_STATS_WALK_SKIP_SINGLE_AREA

If flags are not initialised before beginning a walk the default
set matches the behaviour of previous versions of the library.

Also accept group identifiers as immediate arguments to the
counter, metric, and property functions by adding control
flags to the region and area identifiers passed in.

Region and area properties are mapped to their equivalents for
the group (for example: group size is reported as the sum of
all regions contained in the group). Counter and metric values
are aggregated for the region or group.