ln(1): Sync up with FreeBSD.

ln(1): Sync up with FreeBSD.

* Adds POSIX.1-2008's -L (hardlinks to symbolic links link to the
  symlink target, which stays our default) and -P (hardlinks to
  symbolic links link to the symlink itself) options.

* Adds -F (if the target is a directory, remove it to make the link
  happen) and -w (warns if the source of a symlink does not exist)
  options.

* Some bug fixes I guess. :)

* It also improves the checks for identical files. Our former checks
  were producing kind of confusing results in conjunction with
  'install -C' (i.e., for libraries and includes, if LINKS is used
  from the Makefile) upon repeated installation.
  Reported-by: marino