GLib has g_unix_mount_at (mount_path) already, let's add g_unix_mount_for
(file_path) for whatever path. GLib already contains some private code
for such task. Let's make this code public. This functionality is needed
by GVfs (see Bug 771431) in order to avoid copy-and-pasting.
https://bugzilla.gnome.org/show_bug.cgi?id=772160
