Cross reference tap(4) and tun(4) and include a short explanation as
to how they differ.  This will assist users in selecting which interface
is more appropriate for their purposes.

Approved by:	grog (co-mentor)
MFC after:	2 week
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/userlandenhanced