In Windows 8.1 and later, GetVersionEx does not automatically give the
correct information if a manifest file doesn't explicitly mention we
support that version. This will enable further detection for Windows 8.1
and Windows 10/11 when using windowsversion(), with an easy way to add
future versions. A list of all operating system versions can be found
here:
https://learn.microsoft.com/en-us/windows/win32/sysinfo/operating-system-version

Fixes #21404

refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

fix(ui): re-organize tty fd handling and fix issues

PROBLEM:
exepath("test") should prefer ".bat" when shell=powershell.
Current behavior differs from Vim 8.2.3071.

TEST CASE:
1. in a folder which is in $PATH, create files "test" "test.bat".
   - "(Get-Command test).Path test" returns "test.bat".
2. compare nvim:
     nvim --clean
     :set shell=powershell
     :echo exepath("test")
3. should returns "path\to\test.bat" (before this patch it returns "path\to\test").

SOLUTION:
After this patch, the binary files "text.exe", "test.bat", "test.com"
will be found, but the file "test" (without any extension) will not be
found (matches Vim 8.2.3071). But powershell's `where` and
`Get-Command` _do_ find the extensionless 'test' file.

But Nvim with ":set shell=cmd.exe", doesn't find "test" either (before
and after this patch), even though `where test` returns correct path in
cmd.

- `where` is a program to find files in general, not just executable files.
-`Get-Command` returning extensionless (and thus non-executable) file is
  puzzling even to Chris Dent, [PowerShell expert][1] (asked on
  Discord).

[1]: https://www.amazon.com/Mastering-PowerShell-Scripting-Automate-environment-ebook/dp/B0971MG88X

Co-authored-by: Enan Ajmain <3nan.ajmain@gmail.com>
Helped-by: erw7 <erw7.github@gmail.com>

Fixes #21045

- Use the correct fd to replace stdin on windows (CONIN)
- Don't start the TUI if there are no tty fd (not a regression,
  but makes sense regardless)
- De-mythologize "global input fd". it is just STDIN.

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.