criu: Write pidfile later

Next patch will put non-root task's PID into it, so this
one is preparatory. But, as a bonus, we remove the need
to unlink ths pid file in case of error :)

Risk -- scripts might want to have pidfile, but we already
have CRTOOLS_ROOT_PID environment in them for such cases.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>