User cron

Users can currently run crontabs on individual shellservers, but if the machine gets rebooted, the crontab is lost. It would be nice to have an integrated system so that users can specify jobs to run, and then they get spread out over the shellservers. This system should also offer functionality similar to "at", but it should allow the user to specify that the job gets run on the machine they are currently logged in to.

Implementation
This can be done (somewhat) easily with remctl commands. The user somehow creates a job (possibly just a crontab in a drop folder). When it is time for it to be run, a central server (dionysus) picks a shellserver and tells that machine to run the job. Difficulties include getting the shellserver the _cron keytab.