os: add DirFS

os: add DirFS

It will inevitably be important to be able to pass an operating system
directory to code written to expect an fs.FS.

os.DirFS provides the conversion.

For #41190.

Change-Id: Id1a8fcbe4c7a30de2c47dea0504e9481a88b1b39
Reviewed-on: https://go-review.googlesource.com/c/go/+/243911
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>