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

[Moshe Weitzman]

>  - we can save extra typing and value passing on variable_get calls
>  - we have a hook to reuse when we need a list of variables
>    to present for localization
> 
> adds pluses over just using constants.
> 
> Sure, only using constants for default variable values could be make 
> variable usage better, but I think a hook do more just by being a 
> central place and not a set of unrelated constants.

i'd like to see this centralized in a hook too. we'll be one step closer to 
the hierarchy that adrian proposed long ago: 
http://lists.drupal.org/archives/development/2006-06/msg00309.html