Fix HTTP-requests for files >= 2GB in size

An int is 32-bits on all supported platforms, so this has always been broken. The actual file size/request-length sent would depend on fun 2's complement math (a 32GB file was being truncated to 433MB).

Also fixed some wrong uses of PRIuOFF: off_t is a signed integer, so technically the maximum file size you can request now is 2^63 bytes, which is "big enough".