MSTP update - 5: added bridge port xSTP activation handler

Added #bridge #port level #spanning-tree #activation #handler in MSTP module. On activating the spanning tree on a port it #enable s #xSTP #protocol operations on it. When disabled the xSTP protocols won't be running on the port.

#802-1S #MSTP update 5 ...
[commit 58aa3dd4e0fb05a0b73a930ba3605026e8b5bffb]