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/channel.c b/src/channel.c
index e9c5e12..177ab5a 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -157,7 +157,7 @@
 	if (unlikely(channel_input_closed(chn)))
 		return -2;
 
-	max = buffer_max_len(chn);
+	max = channel_recv_limit(chn);
 	if (unlikely(len > max - buffer_len(chn->buf))) {
 		/* we can't write this chunk right now because the buffer is
 		 * almost full or because the block is too large. Return the
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;