Unlock the startstop mutex while waiting for timer startup

If we start and stop the timer too quickly, the lock can be still held by
the start() function, and the stop() sits deadlocked waiting for it.  As this
delay is primarily to wait for the thread to come up before returning from
start(), it should be safe to release the mutex while waiting to allow the
stop() to take effect.

If this doesn't fix the issue, please reopen the ticket

Fixes #10867

mythtv/libs/libmythbase/mythsignalingtimer.cpp

@@ -48,7 +48,11 @@ void MythSignalingTimer::start(int msec)
         dorun = true;
         MThread::start();
         while (dorun && !running)
+        {
+            locker.unlock();
             usleep(10 * 1000);
+            locker.relock();
+        }
     }
 }