make signal-handler less shit;

previously: threading.Condition to wakeup the actual handler;
exciting chance of heisenbugs / deadlocks (theoretically)

almost went with os.pipe on unix and socketpairs on windows,
but turns out SimpleQueue is perfect and safe for this purpose

SimpleQueue is 3.7+ so use a regular queue on <3.7
(same problems as original approach)

also need dedicated thread for popping the queue on <3.7
to avoid deadlock on most platforms (--sig-thr)

new features:

--logrot-sig sets a signal for immediate log-rotate

--stack-sig sets a signal to dump stack to log/stdout

--reload-sig sets a signal to initiates config-reload
   (was hardcoded to USR1 previously)