MINOR: stream-int: replace si_update() with si_update_both()

The function used to be called in turn for each side of the stream, but
since it's called exclusively from process_stream(), it prevents us from
making use of the knowledge we have of the operations in progress for
each side, resulting in having to go all the way through functions like
stream_int_notify() which are not appropriate there.

That patch creates a new function, si_update_both() which takes two
stream interfaces expected to belong to the same stream, and processes
their flags in a more suitable order, but for now doesn't change the
logic at all.

The next step will consist in trying to reinsert the rest of the socket
layer-specific update code to ultimately update the flags correctly at
the end of the operation.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 54d9810..c8d0a4c 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -56,6 +56,7 @@
 int si_cs_recv(struct conn_stream *cs);
 int si_cs_send(struct conn_stream *cs);
 struct task *si_cs_io_cb(struct task *t, void *ctx, unsigned short state);
+void si_update_both(struct stream_interface *si_f, struct stream_interface *si_b);
 
 /* returns the channel which receives data from this stream interface (input channel) */
 static inline struct channel *si_ic(struct stream_interface *si)