core: avoid loading GIO modules

Turns out the dconf modules is leaky and breaks the valgrind run. In any case,
it's not a good idea to load the modules for the daemon, it just takes time
and memory.

On a Fedora/x86_64 desktop it adds up to 5M to the RSS.

src/main-utils.c

@@ -205,6 +205,9 @@ nm_main_utils_early_setup (const char *progname,
 	 * talking on the session bus.  See rh #588745
 	 */
 	setenv ("GIO_USE_VFS", "local", 1);
+	/* Don't load GIO modules. We just want to load plain files and the
+	 * modules would just take extra memory. */
+	setenv ("GIO_MODULE_DIR", "", 1);
 
 	/*
 	 * Set the umask to 0022, which results in 0666 & ~0022 = 0644.