This reverts behaviour of systemd-run's unit name generation to the status quo ante of #18871: we chop off the ":1." prefix if we can. However, to address the issue that the unique name can overrun we then do what #18871 did as fallback: only chop off the ":" prefix. This way we should have pretty names that look like they always looked in the common case, but in the case of a unique name overrun we still will have names that work. Follow-up for #18871