Add a check "alert" option.

[jimfulton]

Check's get a new option: alert with (initial) options always (default), daytime and never.

always provides current behavior.

never checks never go critical and this never alert.

daytime checks only go critical during daylight hours, where daylight hours are an agent configuration option. They default to 9-20h local (to server). There might be a plumbing issue requiring a change to the internal plumbing interface.

(Note that another way to approach this might be to define an alert type that's included in fault data and the alerter can use to decide how to alert. For pager duty, this might select a different service with different on-call schedules. I think however that the check option might be cleaner because it wouldn't alert until daytime, rather than alerting at night and having to arrange that the alerter somehow deliveres the alert at the right time.)