[net80211] Add initial U-APSD negotiation support.

U-APSD (unscheduled automatic power save delivery) is a power save method
that's a bit better than legacy PS-POLL - stations can mark frames with
an extra flag that tells the AP to leak out more frames after it sends
its own frames rather than needing to send a PS-POLL to get another frame
from the AP.

Now, this code just handles the negotiation bits; it doesn't actually
implement U-APSD. That's up to drivers, and nothing in the tree yet
implements this. I /may/ implement this for ath(4) if I eventually care
enough but right now I plan on just implementing it for firmware offload
based NICs that handle this in the NIC.

I'll commit the ifconfig bit after this and I may have some follow-up
commits as this gets used more by me in local testing.

This should be a glorious no-op for everyone else. If things change
for anyone that isn't fixed by a complete recompile then please reach out
to me.