Improve ability to customize "nav" links below login form.

[dcavins]

In adding a Single Sign-On option to a site, I ran into trouble while trying to add a link to the login form. Flexibility could be introduced by adding an action inside each p.nav in wp-login.php, but there's already duplicated code. So, I've added a function that builds the output and allows plugins to add new links or change the existing links.

Thanks for considering my request.

[dcavins]

Add wp_login_form_nav_links().

[dcavins]

Sample usage: adding an alternative login method.