Start stalwart at boot time, not user login on macOS

[robinvenables]

launchd differentiates between agents and daemons. The main difference is that an agent is run on behalf of the logged in user while a daemon runs on behalf of the root user or any user you specify with the UserName key. Agents are defined in /Library/LaunchAgents and daemons are defined in /Library/LauchDaemons.

I would expect stalwart to run at system boot time, not when a user logs in. This change moves stalwart.mail.plist to /Library/LaunchDaemons and calls launchctl with the up-to-date options and location.

# launchctl load /Library/LaunchAgents/stalwart.mail.plist
Warning: Expecting a LaunchDaemons path since the command was run as root. Got LaunchAgents instead.
`launchctl bootstrap` is a recommended alternative.

[CLAassistant]

All committers have signed the CLA.