Add iframe canvas support for fb friends

The iframe canvas display has different needs from the connect and fbml displays.
This patch change the url and the fb_form_multi_selector() options, using the right url (the theming doesn't change anything here) and adding the iframe destination to avoid have a facebook iframe inside a facebook iframe.

I know that this changes can be done with url_alters or a custom module (I had this way before) but the question is if Facebook for Drupal module plans to support iframe canvas beside fbml and connect (there are some pending patches about this), in other case I don't waste more your time sending patches.