Change the way FreeBSD GID inheritance is hacked.

I believe previous ifdef caused NULL dereference in later zfs_log_create()
on attempt to create file inside directory belonging to ephemeral group
created on illumos, trying to write to log information about GID domain
of the newly created file, inheriting the ephemeral GID.

This patch reuses original illumos SGID code with exception that due to
lack of ID mapping code on FreeBSD ephemeral GID will turn into GID_NOBODY
by another ifdef inside zfs_fuid_map_id().

MFC after: 1 month
Sponsored by: iXsystems, Inc.