[GO-2023-2185] Insecure parsing of Windows paths with a \??\ prefix in path/filepath

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

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.

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

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

In addition, with fix, IsAbs now correctly reports paths beginning with \??\ as
absolute, and VolumeName correctly reports the \??\ prefix as a volume name.

UPDATE: Go 1.20.11 and Go 1.21.4 inadvertently changed the definition of the
volume name in Windows paths starting with \?, resulting in
filepath.Clean(\?\c:) returning \?\c: rather than \?\c:\ (among other effects).
The previous behavior has been restored.