MINOR: channel/buffer: replace b_{adv,rew} with c_{adv,rew}

These ones manipulate the output data count which will be specific to
the channel soon, so prepare the call points to use the channel only.
The b_* functions are now unused and were removed.
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 4e5b90b..9429689 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -341,7 +341,7 @@
 
 		if (bytes32 <= chn->buf->i) {
 			/* OK this amount of bytes might be forwarded at once */
-			b_adv(chn->buf, bytes32);
+			c_adv(chn, bytes32);
 			return bytes;
 		}
 	}
@@ -351,7 +351,7 @@
 /* Forwards any input data and marks the channel for permanent forwarding */
 static inline void channel_forward_forever(struct channel *chn)
 {
-	b_adv(chn->buf, chn->buf->i);
+	c_adv(chn, chn->buf->i);
 	chn->to_forward = CHN_INFINITE_FORWARD;
 }