This is useful for programs that bundle the dokan libraries and want to install
them if not available.  Check the "is_available" attribute to see if dokan is
actually available on the system.

  * move pending-delete cleanup back into Cleanup() function, according to
    Dokan demo code that's where it should be
  * better detection of if/when the drive is ready, as well as reporting
    dokan subproc crashes

Basically, long-format file names the point to a drive are invalid unless they
end with a backslash.  So "\\\\?\\C:" is wrong (and you'll be told it doesn't
exist) while "\\\\?\\C:\\" is right.