xauth: add a configuration string option to be passed to XAuth instances

The configuration string is appended to the XAuth backend name, separated by
a colon. The configuration string is passed untouched to the backend, where
it can change the behavior of the XAuth module.

src/libcharon/plugins/eap_radius/eap_radius_xauth.c

@@ -175,7 +175,8 @@ METHOD(xauth_method_t, destroy, void,
  * Described in header.
  */
 eap_radius_xauth_t *eap_radius_xauth_create_server(identification_t *server,
-												   identification_t *peer)
+												   identification_t *peer,
+												   char *profile)
 {
 	private_eap_radius_xauth_t *this;