By setting exec_stop and stop_timeout (which correspond to
jail(8) attributes exec.stop and stop.timeout), the user
has better control over shutting down a pot.

For "fat" pots this could mean setting

    pot set-attr -A exec_stop -V "/bin/sh /etc/rc.shutdown jail"

for light jails (like nomad controlled using tinirc), this could
point to a simple script that make sure the wrapped process is
stopped gracefully and, in case multiple processes are running
inseide of the pot, make sure they're terminated in the correct
order.

Also:
- Fix a typo that made the nullfs attribute not work.
- Make pot start use _save_params, which makes wrapping
  attributes safer and therefore allows to remove
  a shellcheck exemption.