fix chroot sandbox devices

Description

As part of trying to use BST_FORCE_BACKEND=unix bst shell I discovered integration commands didn't work because /dev/null wasn't functional. This turned out to be caused by two separate bugs in the hardlinks FUSE layer.

Changes proposed in this merge request:

• Add st_rdev to getattr so that device nodes don't get mapped to the "0" device.
• Mount the FUSE layer with -odev so device nodes are usable.

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CHANGELOG/Release Notes

• The platform is consulted for the FUSE mount options. I'm not sure it's the right level of abstraction for future platform FUSE needs so it's marked private with an underscore. It would be premature abstraction for me to make the entirety of how FUSE is mounted platform specific, since I'm not aware of how it would need to differ for other platforms.
• Initial WIP. Will need to be changed to only mount with -odev if used in the chroot sandbox.