Rework GPrivate

Rework GPrivate

 - expose the structure types for GLib internal use only

 - avoid infinite recursion hazards by ensuring that GPrivate never
   calls back into any other part of GLib

 - substantially rework the Windows implementation so that it never
   holds locks, contains no arbitrary limits and doesn't waste
   100*sizeof(void*) per thread

We have to keep the macro hacks for the time being since some code
inside libglib depends on it.