Fix ieee80211_radiotap(9) usage in wireless drivers:

• Alignment issues:
  • Add missing __packed attributes + padding across all drivers; in

most places there was an assumption that padding will be always
minimally suitable; in few places - e.g., in urtw(4) / rtwn(4) -
padding was just missing.

• Add __aligned(8) attribute for all Rx radiotap headers since they can

contain 64-bit TSF timestamp; it cannot appear in Tx radiotap headers, so
just drop the attribute here. Refresh ieee80211_radiotap(9) man page
accordingly.

• Since net80211 automatically updates channel frequency / flags in

ieee80211_radiotap_chan_change() drop duplicate setup for these fields
in drivers.

Tested with Netgear WG111 v3 (urtw(4)), STA mode.

MFC after: 2 weeks