Multi-select instead of group checkboxes on reg form

On the node authoring form, if there are more than 20 groups, the Audience section shows the groups in a multi-select box, rather than checkboxes. This is good. (It happens in og_form_add_og_audience(), on line 1633 of og.module.)

Let's do the same thing with the group checkboxes on the registration form. If more than 20, multi-select.

I can write a patch, unless everyone hates the idea.