Cleanups 6: iostreams: drop extern, hide _Winit

[StephanTLavavej]

• <fstream>, <xlocnum>: _Fiopen() and the _Stofx() family don't need to be declared as extern (that's the default, and we don't say extern when declaring other separately compiled functions).
  • Noticed while implementing Standard Library Modules, where I'll have to add extern "C++" which is different.
• <iostream>: Prevent _Winit from being seen by users.
  • Nothing else mentions this, so we don't need to declare it for users. We still need to export it to preserve bincompat, but I believe that it is completely dead (nothing constructs an object of this type, nor the macroized _UShinit), so I'm adding a comment that it appears to be unused (and thus can be completely removed during vNext).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.