Sanity: don't make a new chroot if -u is specified.
-b: only redirect IOs that are associated with terminals.
Avoid ambiguities for /dev/shm symlink.
Don't remount /tmp dev+exec; instead just create a temporary tmpfs.
Clean up traps: use ' instead of \".
Don't pass stdin to rc.local.
-b: properly pass through stdin if it is not a terminal.