libs/web: fix various issues in cbi option error handling, call validator for missing options to obtain error message

libs/web: rework cbi field validation, let validators return custom error messages, properly handle errors in multiple sections

libiwinfo: support txpwrlist(), freqlist() and countrylist() for radioX pseudodevices in nl80211 backend, implement __gc methamethod

libiwinfo: ensure that country names are ascii, return txpower list even when no channel is set, bump version

libs/uci: prevent concurrent luci-reload instances, write currently processed init script to status file

	- fix compilation of WL backend
	- conditionally compile backends according to selected wifi drivers

- added NL80211 interface for b43 / ath5k / ath9k
- introduces txpower() and encryption() methods
- introduces iwinfo cli frontend for data access
- supports scanning in AP mode (experimental) and through wpa_supplicant