run: when automatically generating names for transient units, use unique bus ID, fallback to random

Previously we used the process ID to generate transient unit names.
However, that is problematic as PIDs get reused easily, and applying
them to remote systems makes little sense.

Fortunately, each bus peer gets a unique, non-reusable ID assigned when
attaching to a bus, hence let's use that, if we can. In some cases we
cannot however, because we connect directly to PID's private socket, and
thus are not a proper bus peer with a unique ID. In that case generate a
random UUID to name the unit after.