New Windows Unicode solution, works on XP

[easyaspi314]

Now uses WriteConsoleW instead of vfwprintf. This is codepage independent and works on Windows XP.

Doing this also avoids the _O_U8TEXT hack.

Also use _vsnprintf instead of vsnprintf, as it is more consistent than vsnprintf in that it doesn't write the null terminator on truncation. Weird.

Some minor cleanup was done as well.

[easyaspi314]

Truly fixes #293.

[sergeevabc]

Err… I’m a mere user without compiler. Could I have a binary to verify the fix, please?

[easyaspi314]

xxhsum-windows-7328e217.zip

[sergeevabc]

Oh, so many binaries, it makes me confused all the time. Right, I confirm that
xxhsum-i686-mingw-w64.exe and xxhsum-i686-mingw32.exe finally work as expected,
and report that xxhsum-i686-msvc2019.exe cannot be launched (XP says “not Win32 app”).

[easyaspi314]

I might have used the wrong filename, or MSVC builds just don't work with XP. I'll test in my VM later.

Edit: Forgot that MSVC sets the minimum version to NT 6.0 a.k.a. Vista. You need to add /link /SUBSYSTEM:CONSOLE,5.1 to support XP.