Convert TFHoliday values to pubvars.

As mentioned in #216, this converts the former TFHoliday enum vars into pubvars that get populated at runtime from gamedata. Since these change semi-often, it's undesirable to request plugins to be recompiled against a new inc each time. With this change, we can just ship new gamedata.

As a side effect, these are no longer *real* constants, so using them as values for switch cases is no longer valid. Very few plugins are affected and the author of the most popular, relevant one, @powerlord, has already been warned of the change.

@asherkin 
@VoiDeD 
@ :japanese_goblin: 
It was intentional due to the different contexts of them - one an early out, and one actual logic.