flux exec should accept a hostlist

[garlick]

Problem: flux exec only understands ranks.

It's useful for running systemd commands like

$ sudo flux exec -r42 systemctl --user list-units --type=service

but ranks are awkward (e.g. one must call flux overlay lookup <hostname>).

[grondo]

Another idea is to just make a flux systemctl wrapper that sets DBUS_SESSION_BUS_ADDRESS appropriately for the flux user so that it could be used locally without needing to run under flux exec (which may seem weird if you don't know why that's required).

However, I agree flux exec should take a hostlist, which is unrelated to the systemd issue.