tun: openbsd: check for interface already being up

tun: openbsd: check for interface already being up

In some cases, we operate on an already-up interface, or the user brings
up the interface before we start monitoring. For those situations, we
should first check if the interface is already up.

This still technically races between the initial check and the start of
the route loop, but fixing that is a bit ugly and probably not worth it
at the moment.

Reported-by: Theo Buehler <tb@theobuehler.org>