Avoid registry collision

Hi,

Replacing a theme function in the registry and keeping the old one is a really good practice but using just "link_default" or "menu_link_default" as old functions names, you are exposing your code to collision with other modules which could need the same as you (Menu Firstchild for example).
Here is a tiny patch without any possible side effects which prefixes theme registry keys with your module name. Thereby, your "(menul_link_default" won't be overrided.

I hope I am clear enough.
Regards.