[MEDIUM] stream interface: add the ->shutw method as well as in and out buffers

Those entries were really needed for cleaner and better code. Using them
has permitted to automatically close a file descriptor during a shut write,
reducing by 20% the number of calls to process_session() and derived
functions.

Process_session() does not need to know the file descriptor anymore, though
it still remains very complicated due to the special case for the connect
mode.
diff --git a/include/proto/stream_sock.h b/include/proto/stream_sock.h
index e104054..45064be 100644
--- a/include/proto/stream_sock.h
+++ b/include/proto/stream_sock.h
@@ -36,6 +36,7 @@
 int stream_sock_data_check_timeouts(int fd);
 int stream_sock_data_update(int fd);
 int stream_sock_data_finish(int fd);
+int stream_sock_shutw(struct stream_interface *si);
 
 
 /* This either returns the sockname or the original destination address. Code