Disease Code Partial Rework

* Cuts /disease variables from 28 to 23 (includes new ones)
* Quite a few variables that survived the cut have been renamed
* A disease may now have more than one spread type (spread_type --> spread_flags)
* Added "infectivity" variable which replaces a prob(65), defaults to 65
* The affected_species variable is now viable_mobtypes, and lists typepaths
* Adds defines for a bunch of stuff

This adds nothing cool for players, but paves the way for something I want to do and should improve the code.