Fix handling of partial and invalid UNC drives in ``ntpath.splitdrive()``, and in
``ntpath.normpath()`` on non-Windows systems. Paths such as '\\server' and '\\' are now considered
by ``splitdrive()`` to contain only a drive, and consequently are not modified by ``normpath()`` on
non-Windows systems. The behaviour of ``normpath()`` on Windows systems is unaffected, as native
OS APIs are used. Patch by Eryk Sun, with contributions by Barney Gale.