(#2251) crontab: avoid duplication of cronjobs

This can be merged, I feel, although it may not yet be perfect. Will need to look into the use of "target" vs "user" for a but, but I'm optimistic.

An open question for me is: Should this semantics apply to all Parsedfile descendants?

I'll see if the bug affects ssh_authorized_key as well, and if so, I think a followup fix can make the behavior universal.

Another point being discussed (hopefully) in Redmine is whether instead of orphaning old cronjobs, we should instead throw some helpful exception so the user has a chance to see the error in their ways.