Correct quota information for sgid directories on FreeBSD

When using netatalk to share many directories, the
quota used to display the size limit for sgid
directories is incorrect.

This comes from the fact that the getfsquota function
in etc/aftp/quota.c grabs the necessary information
from the user's data first and then from any group to
which the user is a member.

If the directory has the setgid or setuid bit set, the
quota information should be derived from that
user/group instead as files created in there will
effectively belong to the user/group owner of the
directory.

The following patch implements this logic (and fixes my
problem). It is valid for netatalk 2.0-alpha2.