journald: dispatch SIGTERM/SIGINT with a low priority

Let's make sure to process all queued log data before exiting, so that
we don't unnecessary lose messages when shutting down.

#1812 (comment)

src/journal/journald-server.c

@@ -1296,10 +1296,22 @@ static int setup_signals(Server *s) {
         if (r < 0)
                 return r;
 
+        /* Let's process SIGTERM late, so that we flush all queued
+         * messages to disk before we exit */
+        r = sd_event_source_set_priority(s->sigterm_event_source, SD_EVENT_PRIORITY_NORMAL+20);
+        if (r < 0)
+                return r;
+
+        /* When journald is invoked on the terminal (when debugging),
+         * it's useful if C-c is handled equivalent to SIGTERM. */
         r = sd_event_add_signal(s->event, &s->sigint_event_source, SIGINT, dispatch_sigterm, s);
         if (r < 0)
                 return r;
 
+        r = sd_event_source_set_priority(s->sigint_event_source, SD_EVENT_PRIORITY_NORMAL+20);
+        if (r < 0)
+                return r;
+
         return 0;
 }