rm: avoid bogus diagnostic for a slash-decorated symlink-to-dir

rm: avoid bogus diagnostic for a slash-decorated symlink-to-dir

These commands would evoke an invalid diagnostic:
    $ mkdir d && ln -s d s && env rm -r s/
    rm: cannot remove 's': Too many levels of symbolic links
remove.c was stripping trailing slashes from "s/" before passing
the name to "rm".  But a trailing slash may change the semantics,
and thus should not be stripped.
* src/remove.c (rm_fts): Do not strip trailing slashes.
* tests/rm/v-slash.sh: Adapt to new expected output.
* gnulib: Update to latest, for an improved fts.c that merely
normalizes trailing slashes.
Reported by Paul Eggert in discussion of http://bugs.gnu.org/12339