Function launchd_plist 

pub fn launchd_plist(plan: &InstallPlan) -> String

Renders the launchd property list for this plan.

KeepAlive is a dictionary rather than <true/> on purpose: the requirement is restart on failure, and a bare KeepAlive also restarts a job that exited cleanly, which would turn a deliberate service stop into a fight with launchd.