core/varlink-manager: report individual job enqueue result if client sets 'more'

core/varlink-manager: report individual job enqueue result if client sets 'more'

One nice property varlink has is that we can nicely report result
of individual operations on each unit, through the 'more' mechanism.
Hence do so for the EnqueueMarkedJobs() method.

Also, apply 502d6f4bc9b96009627f923dcc0ab53e5a181d78 for varlink
method too.

While at it, use lowerCamelCase for method reply fields,
as requested in
https://github.com/systemd/systemd/pull/40365#discussion_r2702344239.