Handle ifname on FreeBSD hosts; if no ifname is given, always start
the search from tap0.  (Simplified/cleaned up version of what has been
in the FreeBSD ports for a long time.)

Signed-off-by: Juergen Lock <nox@jelal.kn-bremen.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>