For clean shutdown in the transceiver we need to cancel and join
running threads for orderly unwinding. Thread cancellation points
already exist, so we just need to be able to call on the threads to
exit out when stopping or shutting down.
Don't error when joining a NULL thread, which would be the case if a
thread was stopped before ever being started to begin with.
Signed-off-by: Tom Tsou <tom@tsou.cc>
Tom Tsou
Olga Bobrova