MINOR: channel: rename buffer_max_len() to channel_recv_limit()

Buffer_max_len() is ambiguous and misleading since it considers the
channel. The new name more accurately designates the size limit for
received data.
diff --git a/src/payload.c b/src/payload.c
index fe9fdda..bc3dedd 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -213,8 +213,8 @@
 	 * all the part of the request which fits in a buffer is already
 	 * there.
 	 */
-	if (msg_len > buffer_max_len(s->req) + s->req->buf->data - s->req->buf->p)
-		msg_len = buffer_max_len(s->req) + s->req->buf->data - s->req->buf->p;
+	if (msg_len > channel_recv_limit(s->req) + s->req->buf->data - s->req->buf->p)
+		msg_len = channel_recv_limit(s->req) + s->req->buf->data - s->req->buf->p;
 
 	if (bleft < msg_len)
 		goto too_short;