Stash the subscriber list when we disconenct from the bus (#35406)

If we unexpectly disconnect from the bus, systemd would end up dropping
the list of subscribers, which breaks the ability of clients like logind
to monitor the state of units.

Stash the list of subscribers into the deserialized state in the event
of a disconnect so that when we recover we can renew the broken
subscriptions.

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Fixes: #8672 #26744

diff --cc src/core/manager-serialize.c
Simple merge

diff --cc src/core/manager.c
Simple merge

diff --cc src/shared/bus-util.c
Simple merge