Don't mount /sys/fs/fuse/connections and /sys/kernel/config

These fail to mount if you don't have the appropriate kernel support,
and this confuses NixOps' ‘check’ command. We should teach NixOps not
to complain about non-essential mount points, but in the meantime it's
better to turn them off.
switch-to-configuration: Use old systemctl to stop units

Otherwise, when switching from systemd 203 to 212, you get errors like:

  Failed to stop remote-fs.target: Bad message
  Failed to stop systemd-udevd-control.socket: Bad message
  ...