run: tweak algorithm for generating unit name from dbus unique name

run: tweak algorithm for generating unit name from dbus unique name

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