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/flt_trace.c b/src/flt_trace.c
index 5d24a41..6b65fe9 100644
--- a/src/flt_trace.c
+++ b/src/flt_trace.c
@@ -440,7 +440,7 @@
 
 static void
 trace_http_reply(struct stream *s, struct filter *filter, short status,
-		 const struct chunk *msg)
+		 const struct buffer *msg)
 {
 	struct trace_config *conf = FLT_CONF(filter);