Add D7-style named implementations of theme_links in page.tpl

In Drupal 7, we have the ability to make a context-specific override of theme_links. In other words, if you want to override the specific instance of theme_links used for the main menu in the page.tpl, a named-implementation allows you to override just that specific instance without affecting all other instances of theme_links.

Highly useful, so we should add it to Zen's page.tpl.