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/include/types/proto_http.h b/include/types/proto_http.h
index 76c8e6b..158f094 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -305,7 +305,7 @@
 struct http_msg {
 	unsigned int msg_state;                /* where we are in the current message parsing */
 	unsigned int flags;                    /* flags describing the message (HTTP version, ...) */
-	struct buffer *buf;                    /* pointer to the buffer which holds the message */
+	struct channel *buf;                   /* pointer to the buffer which holds the message */
 	unsigned int next;                     /* pointer to next byte to parse, relative to buf->p */
 	unsigned int sov;                      /* current header: start of value */
 	unsigned int eoh;                      /* End Of Headers, relative to buffer */