Major improvement to the Xconq kernel

[Jim Kingdon]

> A new path-finding code by Peter Garrone has just been checked in. It uses
> the Astar algorithm and lives in the new source file path.c.

Cool.

So far I found:

* One case of a suboptimal path (went around an independent town the
  wrong way, for one hex more than needed).

* One case where an infantry just sat there in one my my towns, about
  3-5 hexes short of its destination.  It was apparently using up its
  ACP on each turn.  Won't be able to do anything about this unless I
  notice it happening again (as in, I'm not even 100% sure it was
  really a bug I saw).

* One SIGSEGV.  Most interesting part of the stack trace follows.
  Because the default compilation uses -O, it is hard to quickly
  look at the variables at that point.

#5  0x080af4da in point_in_dir (x=6, y=0, dir=135856599, xp=0xbfffde44, yp=0x0)
    at world.c:1459
#6  0x08076069 in attack_blockage (side=0x8381a98, unit=0x8189dcc,
    dirp1=1108574732) at ai.c:2483
#7  0x0807144b in ai_react_to_task_result (side=0x6, unit=0x8189dcc,
    task=0x8416394, rslt=TASK_FAILED) at ai.c:553