logind: factor out polkit checks

logind: factor out polkit checks

Factor out the code to ask polkit for authorization from
method_do_shutdown_or_sleep() into an own function called
verify_shutdown_creds().

This is needed in order to also use the same checks when shutdown
operations are scheduled. For that, it's also necessary to allow
NULL values for that action{,_multiple_sessions,_ignore_inhibit)
arguments, which will suppress the call if no action string is
passed.