Fix potential livelock in do_timer_set

The old code makes all timers interval timers, with the interval equal
to the initial delay, no matter how short the initial delay is (even
1µs). On some systems, this can cause so many signals to be delivered
that the code that resets the timer never gets to run.

The timer is theoretically one-shot; the interval part is only for
if we miss the signal due to a race condition. So we can use a fixed
small delay for the interval.

libmainloop/signal.c

@@ -77,8 +77,10 @@ static void do_timer_set()
         return;
     }
 
-    val.it_interval.tv_usec=val.it_value.tv_usec;
-    val.it_interval.tv_sec=val.it_value.tv_sec;
+    // This really just needs to be a one-shot timer, but it is possible
+    // to miss it if it happens too soon. So add a small interval just
+    // so we _eventually_ wake up.
+    val.it_interval.tv_usec=100000;
 
     if((setitimer(ITIMER_REAL, &val, NULL))){
         had_tmr=TRUE;