Range filter: changed type for total length to off_t.

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.