The high level goal is to render in a better way what caused an
update: coreos#247 (comment)

This gets us for Cockpit:
`Initiated txn DownloadUpdateRpmDiff for client(dbus:1.28 unit:session-6.scope uid:0): /org/projectatomic/rpmostree1/fedora_atomic`
which isn't as good as I'd hoped; I was thinking we'd get `cockpit.service`
but actually Cockpit does invocations as a real login for good reason.

We get a similar result from the CLI.

This makes the logs a bit more useful, but the ultimate goal
here is to write the originating client `id` to the cached update
data, so users know that e.g. `gnome-software` triggered it.