MINOR: channel/buffer: replace buffer_slow_realign() with channel_slow_realign() and b_slow_realign()

Where relevant, the channel version is used instead. The buffer version
was ported to be more generic and now takes a swap buffer and the output
byte count to know where to set the alignment point. The H2 mux still
uses buffer_slow_realign() with buf->o but it will change later.
diff --git a/src/proto_http.c b/src/proto_http.c
index c447929..a4db33a 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -1652,7 +1652,7 @@
 			}
 			if (unlikely(bi_end(req->buf) < b_ptr(req->buf, msg->next) ||
 			             bi_end(req->buf) > req->buf->data + req->buf->size - global.tune.maxrewrite))
-				buffer_slow_realign(req->buf);
+				channel_slow_realign(req);
 		}
 
 		if (likely(msg->next < req->buf->i)) /* some unparsed data are available */
@@ -5139,7 +5139,7 @@
 
 		if (unlikely(bi_end(rep->buf) < b_ptr(rep->buf, msg->next) ||
 		             bi_end(rep->buf) > rep->buf->data + rep->buf->size - global.tune.maxrewrite))
-			buffer_slow_realign(rep->buf);
+			channel_slow_realign(rep);
 
 		if (likely(msg->next < rep->buf->i))
 			http_msg_analyzer(msg, &txn->hdr_idx);
@@ -9517,7 +9517,7 @@
 		 */
 		if (s->req.buf->p > s->req.buf->data &&
 		    s->req.buf->i + s->req.buf->p > s->req.buf->data + s->req.buf->size - global.tune.maxrewrite)
-			buffer_slow_realign(s->req.buf);
+			channel_slow_realign(&s->req);
 
 		if (unlikely(txn->req.msg_state < HTTP_MSG_BODY)) {
 			if (msg->msg_state == HTTP_MSG_ERROR)