settings: return username of agent providing secrets

Mainly for VPN connections to grab the default username from, like
0.8 did when the applet provided the username to NM for user
connections.

src/nm-activation-request.c

@@ -106,6 +106,7 @@ typedef struct {
 static void
 get_secrets_cb (NMSettingsConnection *connection,
                 guint32 call_id,
+                const char *agent_username,
                 const char *setting_name,
                 GError *error,
                 gpointer user_data)