path/filepath: keep the trailing separator on GLOBALROOT device paths

volumeNameLen treats the component after a device prefix as the volume
name, which is what keeps Clean from removing the trailing separator of
a volume root. GLOBALROOT is not a volume: it links to the root of the
NT object namespace, and the volume is named by the two components
after it, so Clean stripped the separator from paths like
\\?\GLOBALROOT\Device\HarddiskVolume1\.

That separator decides what the path refers to: with it the volume's
root directory, without it the raw device. Windows hands these paths
out, notably from the Shadow Copy API.

Consume the two components after GLOBALROOT, as a UNC host and share
already are. Object namespace paths can nest deeper; those are still
not handled.

Fixes #67880

Change-Id: I9780595eadbc8efc77d396edc20908d2c5d48e7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/819060
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>