GNetworkAddress: drop cached addresses on resolver reload

If the resolver reloads (ie, if /etc/resolv.conf changes),
GNetworkAddress needs to re-resolve its addresses the next time it's
enumerated. Otherwise hosts that have different IP addresses inside
and outside a VPN won't work correctly if you hold on to a
GNetworkAddress for them for a long time.

https://bugzilla.gnome.org/show_bug.cgi?id=694181

gio/gnetworkingprivate.h

@@ -34,6 +34,8 @@ gchar *  _g_uri_from_authority             (const gchar      *protocol,
 					    guint             port,
 					    const gchar      *userinfo);
 
+guint64  g_resolver_get_serial             (GResolver        *resolver);
+
 gint g_socket (gint     domain,
                gint     type,
                gint     protocol,