{nl,cfg,mac}80211: let userspace make meshif mesh gate

Allow userspace to set NL80211_MESHCONF_GATE_ANNOUNCEMENTS attribute,
which will advertise this mesh node as being a mesh gate.
NL80211_HWMP_ROOTMODE must be set or this will do nothing.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>