io/fs: allow backslash in ValidPath, reject in os.DirFS.Open
Rejecting backslash introduces problems with presenting
underlying OS file systems that contain names with backslash.
Rejecting backslash also does not Windows-proof the syntax,
because colon can also be a path separator. And we are not
going to reject colon from all names. So don't reject backslash
either.
There is a similar problem on Windows with names containing
slashes, but those are more difficult (though not impossible)
to create.
Also document and enforce that paths must be UTF-8.