MEDIUM: increase chunk-size limit to 2GB-1

Since commit 115acb97, chunk size was limited to 256MB. There is no reason for
such a limit and the comment on the code suggests a missing zero. However,
increasing the limit past 2 GB causes trouble due to some 32-bit subtracts
in various computations becoming negative (eg: buffer_max_len). So let's limit
the chunk size to 2 GB - 1 max.

src/proto_http.c

@@ -2150,7 +2150,7 @@ int http_parse_chunk_size(struct buffer *buf, struct http_msg *msg)
 			break;
 		if (++ptr >= end)
 			ptr = buf->data;
-		if (chunk & 0xF000000) /* overflow will occur */
+		if (chunk & 0xF8000000) /* integer overflow will occur if result >= 2GB */
 			goto error;
 		chunk = (chunk << 4) + c;
 	}