[MEDIUM] reference the current hijack function in the buffer itself

Instead of calling a hard-coded function to produce data, let's
reference this function into the buffer and call it from there
when BF_HIJACK is set. This goes in the direction of more generic
session management code.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index c723651..41f1922 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -66,7 +66,7 @@
 int http_process_request_body(struct session *s, struct buffer *req);
 int process_response(struct session *t);
 
-int produce_content(struct session *s);
+void produce_content(struct session *s, struct buffer *rep);
 int produce_content_stats(struct session *s);
 int produce_content_stats_proxy(struct session *s, struct proxy *px);
 void debug_hdr(const char *dir, struct session *t, const char *start, const char *end);