[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/dumpstats.h b/include/proto/dumpstats.h
index a7c5ab4..bc6bdad 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -45,7 +45,7 @@
 #define STATS_ST_CLOSE 3
 
 int stats_dump_raw(struct session *s, struct uri_auth *uri);
-int stats_dump_raw_to_buffer(struct session *s, struct buffer *req);
+void stats_dump_raw_to_buffer(struct session *s, struct buffer *req);
 int stats_dump_http(struct session *s, struct uri_auth *uri);
 int stats_dump_proxy(struct session *s, struct proxy *px, struct uri_auth *uri);