xfs_scrub: use Unicode skeleton function to find confusing names

xfs_scrub: use Unicode skeleton function to find confusing names

Drop the weak normalization-based Unicode name collision detection in
favor of the confusable name guidelines provided in Unicode TR36 & TR39.
This means that we transform the original name into its Unicode skeleton
in order to do hashing-based collision detection.

The Unicode skeleton is defined as nfd(translation(nfd(string))), which
is to say that it flattens sequences that render ambiguously into a
unambiguous format.  For example, 'l' and '1' can render identically in
some typefaces, so they're both squashed to 'l'.  From the skeletons we
can figure out if two names will look the same, and thereby complain
about them.  The unicode spoofing is provided by libicu, hence the
switch away from libunistring.

Note that potentially confusable names are only worth an informational
warning, since it's entirely possible that with the system typefaces in
use, two names will render distinctly enough that users can tell the
difference.

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