bsd group semantics

The recently added test t1301.17 is failing for me on FreeBSD; the
created directories don't have the GID bit set.

I traced this back to the fact that FORCE_DIR_SET_GID is a no-op on
FreeBSD due to the DIR_HAS_BSD_GROUP_SEMANTICS flag being set. I am not
sure I understand the original reasoning for this flag at all; GID seems
to work just fine on my FreeBSD 6.1 test system.

The original thread is here:

  http://article.gmane.org/gmane.comp.version-control.git/75999

It looks like Alex was having the problem on FreeBSD 4. I'm not sure if
this flag can go away with the new set_shared_perm, or if I need to just
enable on FreeBSD <= 4.

-Peff