Make OFF_T_PF less flaky

[jonathonf]

SIZEOF_OFF_T is a macro that is not defined. This means the use of OFF_T_PF can be flaky which can lead to incorrect value parsing. For example:

sscanf(p, "bytes " OFF_T_PF "-" OFF_T_PF "/", &from, &to);

leads to

zsync_legacy: bytes 21975040-22482943/2388979712
zsync_legacy: failed to parse content-range header 1079950701759516672 - 22482943

Rather than define this macro, this change forces the use of a 64-bit integer via PRIu64 or %llu to support larger range requests. In my testing this makes request range parsing much more robust.

[TheAssassin]

The joy of C...