core: perform clean-up when terminating due to a Unix signal

Since commit 2d700043ab ("core: use g_unix_signal_add() for more
reliable Unix signal handling") we no longer have to stick to
signal-safe functions in the SIGTERM/SIGINT handler.

Use exit() to terminate so that the clean-up functions are ran and code
coverage data gets written into files.

src/main.c

@@ -47,7 +47,7 @@ quit_cb (gpointer user_data)
     if (loop)
         g_idle_add ((GSourceFunc) g_main_loop_quit, loop);
     else
-        _exit (0);
+        exit (0);
     return FALSE;
 }