wtap(4): implement monitor mode and handle radiotap

Implement monitor mode by simply adding IEEE80211_C_MONITOR to ic->ic_cap.

To get additional informations when capturing 802.11 frames, radiotap is inserted by wtap_tx_tap() when TX and wtap_rx_tap() when RX.

There are some type faults in struct wtap_rx_radiotap_header which are mainly mistakenly store unsigned values into signed integers. I have fixed them (wtap(4)) by complying with the types defined in https://www.radiotap.org/fields/defined.

Becuase the struct wtap_rx_radiotap_header comes from ath(4), there may be another patch to fix the type faults in ath(4).

./wtap c 0
./wtap c 1
./wtap c 2

ifconfig wlan0 create wlandev wtap0 wlanmode adhoc ssid test
ifconfig wlan1 create wlandev wtap1 wlanmode adhoc ssid test
ifconfig wlan2 create wlandev wtap2 wlanmode monitor

./vis_map o
./vis_map a 0 1
./vis_map a 0 2
./vis_map a 1 0
./vis_map a 1 2
./vis_map a 2 0
./vis_map a 2 1

ifconfig wlan0 up
ifconfig wlan1 up
ifconfig wlan2 up

tcpdump -i wlan2 -y IEEE802_11_RADIO