GDir: add some glib-private APIs

GDir: add some glib-private APIs

Add a simple UNIX-only API that is used to create a GDir object from a
DIR* that is aquired using opendir() or fdopendir().

This makes it possible to use GDir with openat(), which in turn will
allow use of GDir in the existing GLocalFile implementation of
g_file_measure_disk_usage(), avoiding the current MSVC compatibility
problems there.

Also add an API similar to g_dir_open(), but without the GError handling
(since we want to create a better error message from inside of
glocalfile.c).

Thanks to Chun-wei Fan <fanchunwei@src.gnome.org> for portions of this
patch and for reviews.

https://bugzilla.gnome.org/show_bug.cgi?id=707787