win32: stop leaking GPrivate data

Instead of running the GPrivate destructors from our thread proxy code,
run it from the DllMain handler for the DLL_THREAD_DETACH case.

This should ensure that thread-local data is free at the exit of all
threads -- not just the ones we created for ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=660745

glib/glib-init.h

@@ -31,6 +31,7 @@ GLIB_VAR gboolean g_mem_gc_friendly;
 #ifdef G_OS_WIN32
 #include <windows.h>
 
+G_GNUC_INTERNAL void g_thread_win32_thread_detach (void);
 G_GNUC_INTERNAL void g_thread_win32_init (void);
 G_GNUC_INTERNAL extern HMODULE glib_dll;
 #endif