[freeciv-ai] big unit move cleanup

["Per I. Mathisen" <per@xxxxxxxxxxx>]

This is the previous patch's big brother. It moves all the ai code out of
unithand.c's handle_unit_move_request function and into gotohand.c's
do_unit_goto and three new functions in aitools.c. This is also mostly a
design level patch - it has not been extensively tested. It just seems to
work alright from watching a few AIs slug it out. I have tried as best as
I could to retain previous behaviour, but somehow autogames turn totally
different anyway. A few eyes to help find the reasons for this would be
appreciated.

Also I nixed one serious bug in the small cleanup patch.

Next on the line is do_unit_goto. Someone please explain to me why

  if (punit->pgr) {
    /* we have a precalculated goto route */
    return goto_route_execute(punit);
  }

the above is never executed.

Yours
Per