otus(4) - add initial monitor mode; use lowest rate for EAPOL

The monitor mode stuff is from the openbsd driver, but it doesn't
100% work. It doesn't seem to get all frames for all BSSes.
However, it's enough to at start debugging things. That 0xffffffff
write is /I think/ the RX filter, but I am still not 100% sure about
it all.

Then, whilst here, use the lowest rate for EAPOL frames. This is just
generally a good thing to do.