{nl,mac}80211: add dot11MeshConnectedToMeshGate to meshconf

When userspace is controlling mesh routing, it may have better
knowledge about whether a mesh STA is connected to a mesh
gate than the kernel mpath table.  Add dot11MeshConnectedToMeshGate
to the mesh config so that such applications can explicitly
signal that a mesh STA is connected to a gate, which will then
be advertised in the beacon.

Signed-off-by: Bob Copeland <bobcopeland@fb.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>