when opening a session forward the received cookie to sec-module

That allows to verify that the cookie hasn't been tampered
without relying only on the MAC.

src/ipc.proto

@@ -219,6 +219,8 @@ message cookie
 message sec_auth_session_msg
 {
 	required bytes sid = 1;
+	/* on open */
+	optional bytes cookie = 2;
 	/* on close */
 	optional uint32 uptime = 3;
 	optional uint64 bytes_in = 4;