WIN32: Add an ini setting to specify logpath
WIN32: Move opening logfile from OSystem_SDL::init() to initBackend()

This makes it possible to use the logpath setting from the ini file
for the logfile.
WIN32: Don't create the default Saved Games directory when user has specified a location

There's no need to create those if user doesn't use them. This reduces
ScummVMs footprint, another step closed to make it "portable".