rm: fix empty-name bug introduced with conversion to use fts

rm: fix empty-name bug introduced with conversion to use fts

While "rm ''" would properly fail, "rm F1 '' F2" would fail
to remove F1 and F2, due to the empty string argument.
This bug was introduced on 2009-07-12, via commit 4f73ecaf,
"rm: rewrite to use fts".
* gnulib: Update to latest, for fixed fts.c.
* NEWS (Bug fixes): Describe it.
* tests/rm/empty-name: Adjust for changed diagnostic.
(mk_file): Define, copied from misc/ls-misc.
(empty-name-2): New test, for today's fix.
* lib/xfts.c (xfts_open): Reflect the change in fts_open, now that
it no longer fails immediately when one argument is the empty string.
Assert that the bit flags were not the cause of failure.
* po/POTFILES.in: Remove xfts.c.
* THANKS: Update.
Reported by Ladislav Hagara.