when do_not_email (and other do_not_* fields) are included on profiles, the db value is set to NULL

When you create a profile that includes the "do_not_email" field, that field is saved in the database with NULL rather than 0. This has implications elsewhere in the application - for instance, the AJAX that finds potential people to add to a Quick Email won't turn that person up. Someone else reported this bug here: http://forum.civicrm.org/index.php?topic=26507.0

I've included a patch to fix this. The solution is to pass a default value of "0" for that checkbox when it's not selected, but I also had to add support for passing default values to HTML_QuickForm::Add. Since the underlying functions support setting the values, the rest takes care of itself.