MEDIUM: connection: make conn_sock_shutw() aware of lingering

Instead of having to manually handle lingering outside, let's make
conn_sock_shutw() check for it before calling shutdown(). We simply
don't want to emit the FIN if we're going to reset the connection
due to lingering. It's particularly important for silent-drop where
it's absolutely mandatory that no packet leaves the machine.

src/stream_interface.c

@@ -890,6 +890,7 @@ static void stream_int_shutw_conn(struct stream_interface *si)
 			 * layer to try to signal it to the peer before we close.
 			 */
 			cs_shutw(cs, CS_SHW_NORMAL);
+			conn_sock_shutw(conn);
 
 			/* If the stream interface is configured to disable half-open
 			 * connections, we'll skip the shutdown(), but only if the