Clean up user settings form

When doing the screenshot for http://drupal.org/node/453080#comment-1579256 I realized that we have the "Anonymous user" setting in the site information, when we really should have it under the user settings. It is not a site information property.

So I went on and moved this to the user settings. Since that page is pretty full of fieldsets, I needed to put this in to the initial fieldset, so that it is not completely out of the page flow. So I renamed that to "User identifications settings" which now includes both the anonymous user identification and the user registration. Realized that the registration setting was titled "Public registrations", which seemed problematic, since two of the three options are about how it is not public :) So renamed to more general "User registration". (While I was at it, I reflew the code too).

Finally, at the end of the form, there is this signature and picture support. They both use radio buttons to choose between Enabled and Disabled, while we started an effort long ago to clean up such things and replace them with the more suitable UI component for such an option: a checkbox. So modified these to be checkboxes (the internal value representation did not change, just the UI). There was no explanation whatsoever about the consequence of these settings, so I've added some short explanation. I think part of the reason that there was no explanation was that with radio button groups, the explanation is quite disconnected from the options.

Screenshots to illustrate before and after states: