unicode: reduce the size of utf8data[]

Remove the Hangul decompositions from the utf8data trie, and do
algorithmic decomposition to calculate them on the fly. To store the
decomposition the caller of utf8lookup()/utf8nlookup() must provide a
12-byte buffer, which is used to synthesize a leaf with the
decomposition. This significantly reduces the size of the utf8data[]
array.

Changes made by Gabriel:
  Rebase to mainline
  Fix checkpatch errors
  Extract robustness fixes and merge back to original mkutf8data.c patch
  Regenerate utf8data.h

Signed-off-by: Olaf Weber <olaf@sgi.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>