nl80211: rework hostapd and wpa_supplicant query code

The old nl80211_hostapd_info() used a global static buffer which was not
properly zeroed on subsequent invocations, leading to misreported encryption
values when querying multiple radios or radious with multiple vifs.

Also rework and simplify the control socket code for wpa_supplicant by
splitting status query and scan handling.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>