logind: add ListInhibitors Varlink method

logind: add ListInhibitors Varlink method

The Varlink ListInhibitors method is the counterpart of D-Bus
ListInhibitors. Like its D-Bus counterpart it is zero-filter and streams
the full list of currently registered inhibitors using the
SD_VARLINK_METHOD_MORE pattern, returning InhibitorInfo objects with
Id, What, Who, Why, Mode, UID, PID, and Since fields.

There is no D-Bus GetInhibitor getter to fold in, so no unique-key
filter is introduced here.