path/filepath: consider \\?\c: as a volume on Windows
While fixing several bugs in path handling on Windows,
beginning with \\?\.
Prior to #540277, VolumeName considered the first path component
after the \\?\ prefix to be part of the volume name.
After, it considered only the \\? prefix to be the volume name.
Restore the previous behavior.
Fixes #64028
Change-Id: I6523789e61776342800bd607fb3f29d496257e68
Reviewed-on: https://go-review.googlesource.com/c/go/+/541175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Roland Shoemaker <roland@golang.org>