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
GLocalFile: use GDir for g_file_measure_disk_usage

It turns out that although dirent is available on mingw32 (where the
code was originally tested), it is not usable from MSVC.

Avoid portability problems by just using GDir.

Also, be careful about ensuring that we utf8-format filenames in our
error messages, and leave out the "file://" component since the strings
we're displaying are not URIs (and we don't want to make them URIs since
the extra escaping would reduce legibility).

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