ike: Set default IKE fragment size to 1280

This is the minimum size an IPv6 implementation must support.  This makes
it the default for IPv4 too, which presumably is also generally routable
(otherwise, setting this to 0 falls back to the minimum of 576 for IPv4).

src/libcharon/sa/ike_sa.c

@@ -2979,7 +2979,7 @@ ike_sa_t * ike_sa_create(ike_sa_id_t *ike_sa_id, bool initiator,
 		.flush_auth_cfg = lib->settings->get_bool(lib->settings,
 								"%s.flush_auth_cfg", FALSE, lib->ns),
 		.fragment_size = lib->settings->get_int(lib->settings,
-								"%s.fragment_size", 0, lib->ns),
+								"%s.fragment_size", 1280, lib->ns),
 		.follow_redirects = lib->settings->get_bool(lib->settings,
 								"%s.follow_redirects", TRUE, lib->ns),
 	);