Drop frames that are received unencrypted when WEP is on or when

Drop frames that are received unencrypted when WEP is on or when
WPA is on and RX protection for TA is on.
Keep track of the TX/RX protection for each node when WPA is on.

tested by djm@ (ral+wpa), ckuethe@ (ath-noenc) and krw@ (wpi<->ral+wpa).
hints from bluhm@
has been in snaps for a few days.

pointed out by bluhm@ something like 1 year ago but we did not have
the right infrastructure to fix it properly at that time.

ok deraadt@