Fix interop with older Upstart- and sysvinit-based OSs
While testing steamlinuxruntime!49 (merged) I noticed we had lost interoperability with Ubuntu 14.04.
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bwrap: Don't explicitly mount /dev/pts, /dev/shm
bubblewrap mounts filesystems recursively, so mounting /dev is enough, unless /dev/shm is a symbolic link (which will be handled in a subsequent commit). /dev/pts is assumed not to be a symbolic link: there would be no reason to do so.
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bwrap: Cope with /dev/shm being a symlink to /run/shm
This can be the case on older sysvinit- or Upstart-based Debian-derived distributions, such as Ubuntu 14.04 (with the default Upstart init system) and Debian 9 (with the non-default sysvinit init system).
Under systemd or the versions of sysvinit in Debian >= 10, /dev/shm is a real tmpfs and /run/shm is a symlink to /dev/shm.