rc.d/auditd: set pidfile

auditd creates a pidfile so we should use it for status checks.
This also seems to speed up the frequent onestatus checks used in
tests/sys/audit.

Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D28389

(cherry picked from commit bcc5b2445674e4691853d7533e4873a6712f67ee)