fetch: make -S argument accept values > 2GB

Use strtoll() to parse the argument of the -S option.

FreeBSD has supported 64 bit file offsets for more than 25 years on
all architectures and off_t is a 64 bit integer type for that reason.

While strtol() returns a 64 bit value on 64 LP64 architectures, it
is limit to 32 bit on e.g. i386. The strtoll() function returns a 64
but result on all supported architectures and therefore supports the
possible file lengths and file offsets on 32 bit archtectures.

Reported by: antoine
MFC after: 3 days