Add Linux rfkill support

Add Linux rfkill support

Add a new wpa_supplicant state: interface disabled. This can be used
to allow wpa_supplicant to be running with the network interface even
when the driver does not actually allow any radio operations (e.g.,
due to rfkill).

Allow driver_nl80211.c and driver_wext.c to start while rfkill is in
blocked state (i.e., when ifconfig up fails) and process rfkill
events to block/unblock WLAN.