Add support for files whose canonical paths use UNC prefixes.

[sunfishcode]

On Windows, files on network shares have a canonical path which starts with the UNC "\?" prefix. With these prefixes, the OS no longer handles "." paths and trailing slashes, so handle them manually.

Fixes #2650.

[sunfishcode]

Looking at the test failures, this needs some more work before it's ready.

[sunfishcode]

Windows has some functions which perform canonicalization. I've now submitted https://github.com/retep998/winapi-rs/pull/992 to add bindings to these in winapi, so we can experiment with using those instead of trying to do the canonicalization manually.

[sunfishcode]

Another option is to use windows-rs. That seems like the next thing to try here.

[sunfishcode]

A better path forward for this issue is to change cap-std to use NtCreateFile, which is available as a windows-sys API, so it's stable to use. That way we won't have to do path concatenation and deal with all of these subtle canonicalization and UNC details.