if_otus fixes; add fast-frames support.

Fast-frames:

• include opt_wlan.h ; tsk to not doing it earlier;
• add a tx pending tracking counter for seeing how deep the hardware TX queue is;
• add the frame aging code from if_ath;
• add fast-frames capability to the driver setup.

Bugs:

• free the buffers (and node references) before detaching net80211 state. This prevents a use-after-free in the node free path where we've destroyed net80211 underneath it.