[development] low hanging fruit for Drupal 6: variable defaults

[Moshe Weitzman]

> We should however keep an optional second argument. I don't always use
> the same second argument, and that's intentional.

we now have six years of experience with variable_get(). We rarely rarely 
use this technique. I don't think we have to design for such rare cases. 
Just because we can think of a use case does not mean we have to directly 
support it. My .02

i welcome the centralization of defaults.

-moshe