Change install log location and restrict permissions

[Dean-Coakley]

Proposed changes

• Change install log location from /tmp/ to /var/log/nginx-agent/
• Restrict log file permissions for unprivileged user
• Add install/uninstall CI stage Removed. I recommend adding it back post-open sourcing so it doesn't depend on keys
• Postinstall script output is now appended to the install log
• Suppress excessive stdout output.
• FreeBSD now include preinstall step
• FreeBSD package now creates /var/log/nginx-agent on install instead of on runtime
• FreeBSD no longer errors on uninstall

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)