Fixes to the AI build code

[Hans Ronne]

I have now checked in several fixes to the AI build code that deal with a
common problem where a city would abort building something (e.g. because of
a fight) and then start to build a new unit instead of completing the old
one. In some games, dozens of incomplete units would accumulate inside each
city.

I have also removed the oplayer AI, which was never used the way it was
supposed to (as a development tool for the mplayer).

Hans

Hans Ronne

hronne@pp.sbbs.se