Improved network code for xconq

[Jim Kingdon]

> A number of problems have been fixed, including several sources of
> sync errors and lockups.

Cool!  Interestingly, your changes seem to have made a minor but very
noticeable (to me) change to the user experience of playing against
the AI.  If I am using a fighter to attack, I find that I have to
pause slightly more than before between attack #1 and attack #2.  At
least, I assume this is because of the
lasttime/lastunit/advance_into_cell code that you added.

> I would be interested to hear how the code works in real life now.

I'm playing two players against each other, neither one AI.  Standard
game with sequential play and otherwise default settings.  I'm getting
a "Packet A refers to missing unit #33" warning.  I can email you the
saved games from both players if you want.