core: postpone dbus queue dispatch while API bus setup is pending (#43200)

Since 1166f4472d7669c8008c158a178dd6f76b601fe1 the API bus setup
and the subscriber coldplug happen only once the asynchronous GetId
reply is processed by the event loop. After a daemon-reexec,
manager_dispatch_dbus_queue() runs before subscribers were registered
and consumed send_reloading_done, so the one-shot Reloading(false)
signal was never sent.

Clients that wait for this signal to detect that a reexec finished time out.

Track the pending setup and hold the flag until the reply handler has
re-added the subscriptions.

Also affects v261.2 via backport 26f3717e27f6f08347adf851c605d5fe1f52ac57.