child-create: Add support for multiple key exchanges

It also changes that payloads are built before installing the CHILD_SA
on the responder, that is, the KE payload is generated before keys are
derived, so that key_exchange_t::get_public_key() is called before
get_shared_secret(), or its internal equivalent, which could be relevant
for KE implementations that want to ensure that the key can't be
accessed again after the key derivation.

src/libcharon/sa/ikev2/tasks/child_create.h

@@ -81,13 +81,13 @@ struct child_create_t {
 	void (*use_label)(child_create_t *this, sec_label_t *label);
 
 	/**
-	 * Initially propose a specific DH group to override configuration.
+	 * Initially propose a specific KE method to override configuration.
 	 *
-	 * This is used during rekeying to prefer the previously negotiated group.
+	 * This is used during rekeying to prefer the previously negotiated method.
 	 *
-	 * @param dh_group	DH group to use
+	 * @param ke_method	KE method to use
 	 */
-	void (*use_dh_group)(child_create_t *this, key_exchange_method_t dh_group);
+	void (*use_ke_method)(child_create_t *this, key_exchange_method_t ke_method);
 
 	/**
 	 * Get the lower of the two nonces, used for rekey collisions.

src/libcharon/sa/ikev2/tasks/child_rekey.c

@@ -201,16 +201,16 @@ METHOD(task_t, build_i, status_t,
 	if (!this->child_create)
 	{
 		proposal_t *proposal;
-		uint16_t dh_group;
+		uint16_t ke_method;
 
 		this->child_create = child_create_create(this->ike_sa,
 									config->get_ref(config), TRUE, NULL, NULL);
 
 		proposal = this->child_sa->get_proposal(this->child_sa);
 		if (proposal->get_algorithm(proposal, KEY_EXCHANGE_METHOD,
-									&dh_group, NULL))
-		{	/* reuse the DH group negotiated previously */
-			this->child_create->use_dh_group(this->child_create, dh_group);
+									&ke_method, NULL))
+		{	/* reuse the KE method negotiated previously */
+			this->child_create->use_ke_method(this->child_create, ke_method);
 		}
 	}
 	reqid = this->child_sa->get_reqid_ref(this->child_sa);