systemctl: tweak the "systemctl list-units" output a bit

systemctl: tweak the "systemctl list-units" output a bit

Make the underlining between the header and the body and between the units of
different types span the whole width of the table.

Let's never make the table wider than necessary (which is relevant due the
above).

When space is limited and we can't show the full ID or description string
prefer showing the full ID over the full description. The ID is after all
something people might want to copy/paste, while the description is mostly just
helpful decoration.