UNC and other DOS Device Paths unusable

[CryZe]

The following logic:
https://github.com/bytecodealliance/cap-std/blob/72fdc307184a642eadef22864434109c01d01528/cap-primitives/src/windows/fs/get_path.rs#L18-L22

completely breaks DOS device UNC paths such as:

\\?\UNC\server\share\bar.exe

which would have to be translated to:

\\server\share\bar.exe

in addition to that, other DOS device paths such as:

\\?\Volume{b75e2c83-0000-0000-0000-602f00000000}\Test\Foo.txt

get completely ruined.

There also is a lack of \\.\ handling. I'm also not sure why this gets stripped in the first place, join can handle these paths just fine.

[sunfishcode]

cap-std on Windows implements "openat" by concatenating the path of the base directory with the user-provided relative path, so it gets into weird situations when the user-provided path is in a different format from the base. In theory this is fixable, but I spent some time trying to figure out Windows paths and failed so many times that I'm not eager to try it again. Another possible path forward is to migrate more of the code to NtCreateFile, which some parts of cap-std now use, but not all yet.