glib: Don’t use time(NULL) to get current time

Use either g_get_real_time() or g_date_time_new_now_local(). This means
we don’t need to worry about time_t being 32b in future (the year 2038
problem), and it makes the need for error handling a bit more explicit.
Improve the error handling in several cases.

Based on a patch by Niels De Graef
(https://gitlab.gnome.org/GNOME/glib/merge_requests/142).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1402

gio/tests/gdbus-example-unix-fd-client.c

@@ -74,16 +74,14 @@ on_name_appeared (GDBusConnection *connection,
     }
   else
     {
-      gchar now_buf[256];
-      time_t now;
+      gchar *now_buf = NULL;
       gssize len;
       gchar *str;
+      GDateTime *now = g_date_time_new_now_local ();
 
-      now = time (NULL);
-      strftime (now_buf,
-                sizeof now_buf,
-                "%Y-%m-%d %H:%M:%S",
-                localtime (&now));
+      g_assert_nonnull (now);
+      now_buf = g_date_time_format (now, "%Y-%m-%d %H:%M:%S");
+      g_date_time_unref (now);
 
       str = g_strdup_printf ("On %s, gdbus-example-unix-fd-client with pid %d was here!\n",
                              now_buf,
@@ -95,6 +93,7 @@ on_name_appeared (GDBusConnection *connection,
       g_print ("Wrote the following on server's stdout:\n%s", str);
 
       g_free (str);
+      g_free (now_buf);
       exit (0);
     }
 }