admins request better insight into shutdown progress

[grondo]

During a recent system update, flux shutdown took over 40m with little feedback to indicate progress.

Admins request better insight into shutdown progress. This will give them a better indication if some part of the shutdown process is hung (such as waiting for an unkillable job) or making slow progress (like flux dump).

I think there is already some status set on the systemd unit. I didn't check what is there to see how that could be augmented. When using flux shutdown directly logs may be sent to the user, but perhaps a more distilled status would be appropriate (see also #7206)