Remove the undocumented and ineffective -F option (force hard link

Remove the undocumented and ineffective -F option (force hard link
to directory).  This doesn't reduce functionality but merely results
in a better error message when trying to use the option,
and it simplifies the code.

The -f option first appeared in AT&T Version 7 UNIX (1979), and Keith
Bostic renamed it to -F for 4.3BSD-Reno in 1990 because it conflicted
with System V and POSIX.  Meanwhile, NetBSD, FreeBSD, and DragonFly
removed it, too.

From Tristan Le Guern <tleguern at bouledef dot eu>.
OK guenther@ krw@