seek64

[jaykrell]

The code is a little wierd for seeking past 2GB.
In partcular Posix has fseeko with the apparent intent that o==off_t==long long.

And of course Microsoft has _fseeki64.

My point is, I think we can safely deal with 64bit file offsets, without looping 32bits at a time.

As well, we should consider, maybe, dropping 32bit support?
i.e. long is 64bits on Unix64.

I understand we need some autoconfery. That if _MSC_VER doesn't mean targeting Microsoft runtime.
Maybe there is something we can/should to detect though, i.e. something its headers defines.
#if _WIN32 could imply the Windows runtime perhaps.