A-star fixes

Will obey the maxnodes parameter if it's provided (although perhaps not
in the way that it was originally meant?) The only one that uses it is
disease, and it wasn't an issue there, but at least in future it won't
be a problem.

Also slightly increased the maximum loop count from 1500 to 1800, on the
off-chance that the previous value was pessimistically low. In the
initial station this doesn't go past 1000, but if bots understand doors
their paths might get squiggly.