Controlling KeepChildren param from Ticket.XML to retain valid services of invalid parents in listings

Added Ticket::Service::KeepChildren config to Ticket.XML and modified Kernel\System\Ticket.pm TicketServiceList() and Kernel\Modules\AgentTicketSearch.pm Run() to pass the KeepChildren parameter to Kernel\System\Service.pm ServiceList() as per the user established config value.