This is useful for passing directory file descriptors over UNIX
sockets or to child processes to avoid TOCTOU vulnerabilities.

The implementation follows the Dir.chdir code.

This will raise NotImplementedError on platforms not supporting
both fchdir and dirfd.

Implements [Feature #19347]

HOMEPATH is set to "\WINDOWS\system32" when running per "runas" session.
This directory is not writable by ordinary users, leading to errors with many ruby tools.
Also config files in the home directory are not recognized.

Still keeping HOME at first which is not used by native Windows,
but by ruby specs and by MSYS2 environment.

Dir.home returns an UTF-8 string since ruby-3.0, but the actual
encoding of the bytes was CP_ACP or CP_OEMCP.
That led to invalid bytes when calling Dir.home with an unicode
username.

I don't come up with a way to fix it right away. We'd need some
experiments on a pull request.