mac_seeotheruids(4) on FreeBSD
The text 3b0c6e2ed7a1, as FreeBSD 6.3-RELEASE, FreeBSD 6.2-RELEASE, FreeBSD 6.1-RELEASE, FreeBSD 6.0-RELEASE, FreeBSD 5.5-RELEASE, FreeBSD 5.4-RELEASE, FreeBSD 5.3-RELEASE shipped it.
simple policy controlling whether users see other users