[CF-Devel] Poison fog implemented and in repository

[Peter Mardahl]

I did a poison fog spell.
It tracks and leaves behind a trail of cloud

the trail is opaque and cannot be seen through
the poisoning ignores any invisible monster (i.e., one already
in the cloud) and goes after new ones

the poison fog effect is slower than ball-lightning or divine wind,
but it uses the same tracking algorithm.

PM