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/include/proto/action.h b/include/proto/action.h
index dcfdeac..19312db 100644
--- a/include/proto/action.h
+++ b/include/proto/action.h
@@ -44,7 +44,8 @@
 	return NULL;
 }
 
-static inline void action_build_list(struct list *keywords, struct chunk *chk)
+static inline void action_build_list(struct list *keywords,
+				     struct buffer *chk)
 {
 	struct action_kw_list *kw_list;
 	int i;