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.

Fixes #44166.

Change-Id: Iac7a9a268025c1fd31010dbaf3f51e1660c7ae2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/290709


TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>