MINOR: http-ana: Add a function for forward internal responses

Operations performed when internal responses (redirect/deny/auth/errors) are
returned are always the same. The http_forward_proxy_resp() function is added to
group all of them under a unique function.
diff --git a/include/proto/http_ana.h b/include/proto/http_ana.h
index 57fd881..2b35388 100644
--- a/include/proto/http_ana.h
+++ b/include/proto/http_ana.h
@@ -51,6 +51,7 @@
 void http_reply_and_close(struct stream *s, short status, const struct buffer *msg);
 void http_return_srv_error(struct stream *s, struct stream_interface *si);
 struct buffer *http_error_message(struct stream *s);
+int http_forward_proxy_resp(struct stream *s, int final);
 
 struct http_txn *http_alloc_txn(struct stream *s);
 void http_init_txn(struct stream *s);