802.11s: fix authsae support in netifd

This patch implements support for 802.11s protected mesh wireless networks (using authsae) in the netifd framework.

Until meshd-nl80211 implements a proper -P option for the PID file, this uses shell backgrounding in order to be able to get the PID for the process.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40497 3c298f89-4303-0410-b956-a3cf2f4a3e73