From the original OpenBSD commit message:
restore the traditional behavior of -f implying -a; apparently Keith
Bostic forgot to restore it when the -f flag was put back on 2nd of
September 1989, after being removed on 16th of August as a
consequence of issues getting it working over NFS, so deviation from
traditional UNIX behavior in all BSDs looks like an historical
accident; as a side effect, this change accommodates behavior of
this option to IEEE Std 1003.1-2008 (``POSIX.1'').
joint work with jmc@ (who found the inaccuracy in our
implementation), schwarze@ (who provided a detailed tracking of
historical facts) and millert@
Submitted by: Igor Sobrado
Discussed with: mckusick
Obtained from: OpenBSD project
MFC after: 2 weeks