ike-init: Switch to an alternative config if proposals don't match

This way we don't rely on the order of equally matching configs as
heavily anymore (which is actually tricky in vici) and this also doesn't
require repeating weak algorithms in all configs that might potentially be
selected if there are some clients that require them.

There is currently no ordering, so an explicitly configured exactly matching
proposal isn't a better match than e.g. the default proposal that also
contains the proposed algorithms.

src/libcharon/sa/ike_sa.c

@@ -674,6 +674,7 @@ METHOD(ike_sa_t, get_ike_cfg, ike_cfg_t*,
 METHOD(ike_sa_t, set_ike_cfg, void,
 	private_ike_sa_t *this, ike_cfg_t *ike_cfg)
 {
+	DESTROY_IF(this->ike_cfg);
 	ike_cfg->get_ref(ike_cfg);
 	this->ike_cfg = ike_cfg;
 }