Range filter: changed type for total length to off_t. Total length of a response with multiple ranges can be larger than a size_t variable can hold, so type changed to off_t. Previously, an incorrect Content-Length was returned when requesting more than 4G of ranges from a large enough file on a 32-bit system. An additional size_t variable introduced to calculate size of the boundary header buffer, as off_t is not needed here and will require type casts on win32. Reported by Shuxin Yang, http://mailman.nginx.org/pipermail/nginx/2017-July/054384.html.