varlink: add exit-on-idle logic for Varlink server

varlink: add exit-on-idle logic for Varlink server

This adds a logic that if enabled ensures sd_event_exit() is called
whenever the varlink connection count hits zero.

This is useful for implementing pure Varlink services (i.e. services
whose only job is to serve Varlink requests), that shall run only as
long as needed, i.e. as long as at least one request is being served.