logind: switch to sd_bus_track helper

logind: switch to sd_bus_track helper

Let logind use the sd_bus_track helper object to track the controllers of
sessions. This does not only remove quite some code but also kills the
unconditional matches for all NameOwnerChanged signals.

The latter is something we should never ever do, as it wakes up the daemon
every time a client connects, which doesn't scale.