mkfs: remove notion of config "type"

mkfs: remove notion of config "type"

The whole notion of a defaults "type" led to complications that
simply doesn't need to be there.  If a configuration file was
used for defaults, simply state that fact and carry on.

This also removes ambiguity for config files specified as
a relative path by calling realpath.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>