[MEDIUM] make the http server error function a pointer in the session

It was a bit awkward to have session.c call return_srv_error() for
HTTP error messages related to servers. The function has been adapted
to be passed a pointer to the faulty stream interface, and is now a
pointer in the session. It is possible that in the future, it will
become a callback in the stream interface itself.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 3addf3f..72a5e3c 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -84,7 +84,7 @@
 		      struct hdr_ctx *ctx);
 void http_sess_log(struct session *s);
 void perform_http_redirect(struct session *s, struct stream_interface *si);
-void return_srv_error(struct session *s, int err_type);
+void http_return_srv_error(struct session *s, struct stream_interface *si);
 
 #endif /* _PROTO_PROTO_HTTP_H */