logcheck: add some options to ease setting up ignore-rules

The special handling for cronjobs should probably move to the cron
module (logcheckIgnore = bool option) in the future, as it's more
natural to just declare a cronjob, and mark it as "log-ignored",
instead of adding cronjobs through logcheck.

But as systemCronjobs is not an attrset yet (just simple strings),
this would require adding an attrset for cronjobs or parsing strings
in the nix language to get hold of the cron-user and command.

So for now, I keep the interface within logcheck's module.