bin/check_perms is wrong for *BSD

[Schnoebe]

bin/check_perms _assumes_ that you're running on an OS
that requires the setgid bit on directories to have the
directory ownership information propagate to files
created in that directory.

This is wrong for BSD derived operating systems.

bin/check_perms should be modified to understand this,
rather than operating on false assumptions.

I've attached a patch that solves the problem. Forgive
the style (if any) as this is the first time I've
wacked on Python.

[http://sourceforge.net/tracker/index.php?func=detail&aid=855096&group_id=103&atid=100103]