path/filepath: recognize \??\ as a Root Local Device path prefix.

On Windows, a path beginning with \??\ is a Root Local Device path equivalent
to a path beginning with \\?\. Paths with a \??\ prefix may be used to
access arbitrary locations on the system. For example, the path \??\c:\x
is equivalent to the more common path c:\x.

The filepath package did not recognize paths with a \??\ prefix as special.

Clean could convert a rooted path such as \a\..\??\b into
the root local device path \??\b. It will now convert this
path into .\??\b.

IsAbs did not report paths beginning with \??\ as absolute.
It now does so.

VolumeName now reports the \??\ prefix as a volume name.

Join(`\`, `??`, `b`) could convert a seemingly innocent
sequence of path elements into the root local device path
\??\b. It will now convert this to \.\??\b.