cfg80211/mac80211: add mesh join/leave commands

Instead of tying mesh activity to interface up,
add join and leave commands for mesh. Since we
must be backward compatible, let cfg80211 handle
joining a mesh if a mesh ID was pre-configured
when the device goes up.

Note that this therefore must modify mac80211 as
well since mac80211 needs to lose the logic to
start the mesh on interface up.

We now allow querying mesh parameters before the
mesh is connected, which simply returns defaults.
Setting them (internally renamed to "update") is
only allowed while connected. Specify them with
the new mesh join command instead where needed.

In mac80211, beaconing must now also follow the
mesh enabled/not enabled state, which is done
by testing the mesh ID.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>