Strip down the Windows ll[io]fstream implementations to constructors

and open() methods. The only remaining value added by ll[io]fstream over
std::[io]stream is proper handling of non-ASCII pathnames, which can be done
by deriving from std::[io]stream, converting pathname strings and passing them
to the corresponding base-class methods.
This is only necessary on Windows. On Posix, ll[io]fstream are already
typedefs for std::[io]fstream.
This change removes a significant volume of cruft from llfile.{h,cpp}.