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/plugins/kernel_libipsec/kernel_libipsec_ipsec.c

@@ -231,7 +231,8 @@ static void expire(uint8_t protocol, uint32_t spi, host_t *dst, bool hard)
 METHOD(kernel_ipsec_t, get_features, kernel_feature_t,
 	private_kernel_libipsec_ipsec_t *this)
 {
-	return KERNEL_REQUIRE_UDP_ENCAPSULATION | KERNEL_ESP_V3_TFC;
+	return KERNEL_REQUIRE_UDP_ENCAPSULATION | KERNEL_ESP_V3_TFC |
+		   KERNEL_SA_USE_TIME;
 }
 
 METHOD(kernel_ipsec_t, get_spi, status_t,