Conditionally install /etc/rc.d/audit* based on ${MK_AUDIT}

/usr/sbin/audit(dist)?d are only installed if ${MK_AUDIT} == yes. Their
supporting scripts should only be installed in those instances as well.

Submitted by: ngie
Reviewed by: emaste
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd/pull/242