mount-chroot doesn't cope with swapped /dev/shm and /run/shm

[Adam Conrad]

Somewhere in the last cycle or two, /dev/shm and /run/shm were swapped, as was the symlink. This wasn't noticed on lp-buildd as all my old chroots were upgraded and didn't inherit the swap, but my new arm64 chroot was built with the inverse order and mount-chroot's mounting bits failed to setup a shm environment.

For now, I've made the arm64 chroot look like the others, but we need to fix this to test /dev/shm and /run/shm for linkiness versus directoryness and mount the one that's a directory, so we can support both old and new style setups.