- Fix various #include issues

 - Change #error to #warning for the EXTERNAL authentication mechanism.
   It is not clear if this should work on Win32 at all.

 - Call close() before unlink() for the SHA1 keyring

 - Change #error to #warning so we don't forget to do
   permission checking of the .dbus-keyrings directory

 - Use Win32 SID for the SHA1 auth mech

 - Apparently we can't use word 'interface' as an identifier

 - Implement a _g_dbus_win32_get_user_sid() function. For now it's
   private. Don't know if it should be public somewhere. Maybe in
   a future GCredentials support for Win32? I don't know.

 - GFileDescriptorBased is not available on Win32. So avoid using
   it in GLocalFile stuff. Now, Win32 still uses GLocalFile + friends
   (which works with file descriptors) so expose a private function
   to get the fd for an OutputStream so things still work.

 - Fixup gio.symbols

 - Fixup tests/gdbus-peer.c so it builds

With this, at least things compile and the gd...