Update FreeBSD rc.d script

[CVanF5]

Proposed changes

Updates the FreeBSD rc.d script to use the standard FreeBSD rc.subr template. This reduces the complexity of the file and fixes some errors that occurs when running service nginx-agent restart

freebsd@testenv-3dd5fc2a-data-1:~ $ sudo service nginx-agent restart
nginx_agent already running?  (pid=3902).
freebsd@testenv-3dd5fc2a-data-1:~ $ sudo service nginx-agent restart
cat: /var/run/nginx_agent.pid: No such file or directory
kill: illegal process id: 
freebsd@testenv-3dd5fc2a-data-1:~ $ echo $?
1

Updated script

freebsd@testenv-3dd5fc2a-data-1:~ $ sudo service nginx-agent restart
Stopping nginx_agent.
Waiting for PIDS: 4081, 4081.
Starting nginx_agent.
freebsd@testenv-3dd5fc2a-data-1:~ $ echo $?
0

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

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[CVanF5]

Thanks for the approval! I'm not authorized to merge, so I thought I'd just leave this note in case someone else feels like pushing the button.