systemctl: better width and alignment of fields

systemctl: better width and alignment of fields

Provide nicer output by taking advantage of the available columns.

Expand UNIT and DESCRIPTION if possible. Economize the space:
 - never give UNIT more width than it can use.
 - if there's a shortage of space, give 25 columns to UNIT, even
   if it would result in overflowing the total columns. The field would not
   be useful below width 25.
 - then if possible, assign up to 25 columns to DESCRIPTION.
 - then split any remaining space evenly between UNIT and DESCRIPTION.

Keep the columns aligned even with '--full'.