logind: support deserializing session leader through pidfdid

logind: support deserializing session leader through pidfdid

People make weird assumptions around state preservation and
expect logind to be stoppable. While this is realistically
not OK we can probably improve things a little.

This complements f01d8658a3a57d05a5156aefd32d8137c3ee3996 and
adds support for deserializing the LEADER_PIDFDID= field.
We still prioritize pidfd if got one from fdstore (as with
service_notify_message_parse_new_pid() in pid1), but otherwise
this should make logind restart more robust when fdstore
gets spuriously cleared.

Fixes #39437