kernel-netlink: Add options to enable parallel Netlink queries explicitly

As under vanilla Linux the kernel can't handle parallel dump queries and returns
EBUSY, it makes not much sense to use them. Disable parallel queries by default
to basically restore original behavior, improving performance.

src/libhydra/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -2711,7 +2711,9 @@ kernel_netlink_ipsec_t *kernel_netlink_ipsec_create()
 		fclose(f);
 	}
 
-	this->socket_xfrm = netlink_socket_create(NETLINK_XFRM, xfrm_msg_names);
+	this->socket_xfrm = netlink_socket_create(NETLINK_XFRM, xfrm_msg_names,
+				lib->settings->get_bool(lib->settings,
+					"%s.plugins.kernel-netlink.parallel_xfrm", FALSE, lib->ns));
 	if (!this->socket_xfrm)
 	{
 		destroy(this);