`std.os.getFdPath` is very platform-specific and can be used to query
the OS for a canonical path to a file handle. Currently supported hosts
are Linux, macOS and Windows.

`std.fs.Dir.realpath` (and null-terminated, plus WTF16 versions) are
similar to `std.os.realpath`, however, they resolve a path wrt to this
`Dir` instance.

If the input pathname argument turns out to be an absolute path, this
function reverts to calling `realpath` on that pathname completely
ignoring this `Dir`.