MONITOR: Split up check_services()
    
    Let's split up check_services() function and create a check_service()
    function, that receives a single service name as parameter and checks
    whether the service is a known service or not.
    
    This new function will be used in order to check the socket activated
    services.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2243
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>