ap: remove unused device whitelists
ap: do not handle sigchlds anymore

The previous handling was not enough on its own (what if hostapd or
dhclient failed?). It's better to leave it unimplemented until there's
time to implement it properly.
ap: inline read_puavo_files() and assume /etc/puavo/wlan/ssid exists
ap: inline validate_config()
ap: ignore terminating signals during exit cleanup
ap: simplify on_exit()
ap: rewrite get_amode_ifaces() to find_ifaces(hw_mode)
ap: args parsing in a separate function
ap: replace _fail() and _warn() with general purpose _log()
ap: remove status reporting function from common library
ap: simplify cleanup
ap: move config reading to a separate function
ap: refactor a bit
ap: enhance comments
ap: separate the bridge setup and vtund up event
ap: fix bssid masking