Don't expose host /dev/shm with --device=all (fixing debian problems)

--device=all really means the device nodes, we should not expose the host
shared memory objects.

This change incidentally fixes issues with --device=all on debian (#2136)
where /dev/shm is a symlink to /run/shm, which doesn't exist in the sandbox.

Closes: #2160
Approved by: alexlarsson
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