dmstats: add create --filemap

dmstats: add create --filemap

Add a new option to the create command to create regions that map the
extents of a file:

  # dmstats create --filemap /path/to/file
  /path/to/file: Created new group with 10 region(s) as group ID 0.

When performing a --filemap no device argument is required (and
supplying one results in error) since the device to bind to is implied
by the file path and is obtained directly from an fstat().

Grouping may be optionally disabled by the --nogroup switch: in this
case the command will report each region individually:

  # dmstats create --nogroup --filemap /path/to/file
  /path/to/file: Created new region with 1 area as region ID 0.
  /path/to/file: Created new region with 1 area as region ID 1.
  /path/to/file: Created new region with 1 area as region ID 2.

When grouping regions the group alias is automatically set to the
basename (as returned by dm_basename()) of the provided file.

This can be overridden to a user-defined value at the command line by
use of the --alias option.

If grouping is disabled no alias can be set.

Use of offset and subdivision options (--start, --length, --segments,
--areas, --areasize).

Setting aux_data and histograms for groups is possible but is not
currently implemented.