Use systemd-tmpfiles to create the private tmp mount namespace root
dir (/tmp/snap-private-tmp) on boot as owned by root with restrictive
permissions. We can use this as a known location to then create per-snap
private tmp mount namespace dirs (/tmp/snap-private-tmp/snap.$SNAP_INSTANCE)
etc.
Signed-off-by: Alex Murray <alex.murray@canonical.com>
