MAJOR: chunks: replace struct chunk with struct buffer

Now all the code used to manipulate chunks uses a struct buffer instead.
The functions are still called "chunk*", and some of them will progressively
move to the generic buffer handling code as they are cleaned up.
diff --git a/src/filters.c b/src/filters.c
index a4005ed..3956667 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -667,7 +667,7 @@
  * decides to stop the HTTP message processing.
  */
 void
-flt_http_reply(struct stream *s, short status, const struct chunk *msg)
+flt_http_reply(struct stream *s, short status, const struct buffer *msg)
 {
 	struct filter *filter;