Go around functionality added.

If MAV_CMD_DO_GO_AROUND is received when the plane is in the LAND_APPROACH flight stage then, if possible, the mission proceeds beyond the landing point (next mission item) to what is assumed to be a mission segment for performing a go around.

Also added: MAV_CMD_CONTINUE_AND_CHANGE_ALT, which is a new mission item that will continue following the present course until the specified altitude is reached.