rm: new option --dir (-d) to remove empty directories

rm: new option --dir (-d) to remove empty directories

Add new option to rm (-d/--dir), which allows removal of
empty directories, while still safely disallowing removal
of non-empty ones.

This improves compatibility with Mac OS X and BSD systems,
which honor the -d option.

* src/remove.c (rm_fts): Remove empty directories when requested.
* src/remove.h (rm_options) [remove_empty_directories]: New member.
* src/rm.c (long_opts, usage, main): Update usage and option parsing.
(rm_option_init): Initialize the new member.
* src/mv.c (rm_option_init): Initialize the new member.
* tests/rm/d-1: New test case - successfully delete empty dir.
* tests/rm/d-2: New test case - refuse to delete nonempty dir.
* tests/Makefile.am (TESTS): Add them.