directory: limit deep scount (folder contents and size) to one filesystem

When getting the size of the root directory, nautilus was descending
into other filesystems including /proc, causing it to report nonsensical
sizes.
Store the fsid of the starting directory, and do not recurse into
other mount points.

https://bugzilla.gnome.org/show_bug.cgi?id=629394
https://bugs.launchpad.net/ubuntu/+source/nautilus/+bug/585472