Fix handling of devices w/o radiotap support:

Fix handling of devices w/o radiotap support:
o do not attach DLT_IEEE802_11_RADIO unless both tx and rx headers are
  present; this is assumed in the capture code paths
o verify the above with asserts in ieee80211_radiotap_{rx,tx}
o add missing checks for active taps before calling ieee80211_radiotap_rx