add activation timeout in bsc_handover_start()
bsc_handover_start() calls rsl_chan_activate_lchan() without detecting a missing ACK/NACK.
bsc_handover_start() should have an lchan->act_timer and mark the channel broken if timed out.
See rsl_rx_chan_rqd(), which calls rsl_chan_activate_lchan() with a timeout.
This issue was raised by hfreyther in https://gerrit.osmocom.org/#/c/210/1/openbsc/src/libbsc/abis_rsl.c@479