kernel-interface: Add feature to indicate if query_sa() returns last use time

Currently supported by libipsec and PF_KEY on macOS (FreeBSD, like Linux,
reports the time the SA was first used in sadb_lifetime_usetime - it also
triggers rekeyings based on that, which Linux doesn't, it also triggers
them if an SA is never used).

src/libcharon/kernel/kernel_interface.h

@@ -79,6 +79,8 @@ enum kernel_feature_t {
 	KERNEL_NO_POLICY_UPDATES = (1<<3),
 	/** IPsec backend supports installing SPIs on policies */
 	KERNEL_POLICY_SPI = (1<<4),
+	/** IPsec backend reports use time per SA via query_sa() */
+	KERNEL_SA_USE_TIME = (1<<5),
 };
 
 /**
@@ -202,7 +204,11 @@ struct kernel_interface_t {
 						  kernel_ipsec_update_sa_t *data);
 
 	/**
-	 * Query the number of bytes processed by an SA from the SAD.
+	 * Query the number of bytes and packets processed by an SA from the SAD.
+	 *
+	 * Some implementations may also return the last use time (as indicated by
+	 * get_features()). This is a monotonic timestamp as returned by
+	 * time_monotonic().
 	 *
 	 * @param id			data identifying this SA
 	 * @param data			data to query the SA
@@ -247,11 +253,12 @@ struct kernel_interface_t {
 	 * Query the use time of a policy.
 	 *
 	 * The use time of a policy is the time the policy was used
-	 * for the last time.
+	 * for the last time. This is a monotonic timestamp as returned by
+	 * time_monotonic().
 	 *
 	 * @param id			data identifying this policy
 	 * @param data			data to query the policy
-	 * @param[out] use_time	the monotonic timestamp of this SA's last use
+	 * @param[out] use_time	the monotonic timestamp of this policy's last use
 	 * @return				SUCCESS if operation completed
 	 */
 	status_t (*query_policy)(kernel_interface_t *this,