MINOR: channel: rename bi_full to channel_full as it checks the whole channel

Since the function takes care of the forward count and involves more than
buffer knowledge, rename it.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 14e2cd5..2f3400d 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -727,7 +727,7 @@
 
 		b->flags |= BF_WRITE_PARTIAL;
 
-		if (likely(!bi_full(b)))
+		if (likely(!channel_full(b)))
 			b->flags &= ~BF_FULL;
 
 		if (!b->buf.o) {
@@ -1080,7 +1080,7 @@
 		b->flags |= BF_READ_PARTIAL;
 		b->total += ret;
 
-		if (bi_full(b)) {
+		if (channel_full(b)) {
 			/* The buffer is now full, there's no point in going through
 			 * the loop again.
 			 */