rm: do not skip files upon failure to remove an empty dir

rm: do not skip files upon failure to remove an empty dir

When removing a directory fails for some reason, and that directory
is empty, the rm_fts code gets the return value of the excise call
confused with the return value of its earlier call to prompt,
causing fts_skip_tree to be called again and the next file
that rm would otherwise have deleted to survive.

* src/remove.c (rm_fts): Ensure we only skip a single fts entry,
when processing empty dirs.  I.e. only skip the entry
having successfully removed it.
* tests/rm/empty-immutable-skip.sh: New root-only test.
* tests/local.mk: Add it.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/44883