/quit now exits from the event loop

Before this change issuing `/quit` directly called `exit(0)` and did not
invoke all the graceful shutdown routines. Now we first try to exit from
the event loop, which includes cleaning up everything.
In case the event loop is stuck for some reason, you could try to issue a
second `/quit`, which will then directly call `exit(0)`.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/command/cmd_funcs.c

@@ -1328,7 +1328,8 @@ gboolean
 cmd_quit(ProfWin* window, const char* const command, gchar** args)
 {
     log_info("Profanity is shutting down…");
-    exit(0);
+    if (prof_set_quit())
+        exit(0);
     return FALSE;
 }