REORG/MAJOR: use "struct channel" instead of "struct buffer"

This is a massive rename. We'll then split channel and buffer.

This change needs a lot of cleanups. At many locations, the parameter
or variable is still called "buf" which will become ambiguous. Also,
the "struct channel" is still defined in buffers.h.
diff --git a/src/acl.c b/src/acl.c
index e2e72ce..b4c5908 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -120,7 +120,7 @@
 {
 	int hs_len;
 	int hs_type, bleft;
-	struct buffer *b;
+	struct channel *b;
 	const unsigned char *data;
 
 	if (!l4)
@@ -324,7 +324,7 @@
                         const struct arg *args, struct sample *smp)
 {
 	int hs_len, ext_len, bleft;
-	struct buffer *b;
+	struct channel *b;
 	unsigned char *data;
 
 	if (!l4)