Add ServiceState type

[atc0005]

Currently used by the atc0005/check-cert project:

// ServiceState represents the status label and exit code for a service check.
type ServiceState struct {

	// Label maps directly to one of the supported Nagios state labels.
	Label string

	// ExitCode is the exit or exit status code associated with a Nagios
	// service check.
	ExitCode int
}