net/mosquitto: update rc.d script to fix mosquitto_pidfile override

The default variables were being set before load_rc_config was invoked.

jrm@FreeBSD.org did most of this work and all credit goes to him.
I only found the problem. He helped tremendously.

PR: 258089
Reported by: dvl
Approved by: joe@thrallingpenguin.com (maintainer)
Obtained from: jrm