Validate author fields with #element_validate

Both approaches seem valid, however this approach allows for a bit more re-use which is nice.

I'd let the validate handler focus on validating the username -- at least for now -- so no need to return anything extra IMO. It wouldn't be an elegant API, IMO.

#564702: The user module should use #element_validate is passing now, and also implements a (slightly smarter) user_name_element_validate() why not merge the two?

Applied the patch and tested it locally and the patch works as advertised.

Some comments/questions after reviewing:

1. + * Form elements should additionally set #required => TRUE when a valid user is
   + * required. Otherwise, it is assumed that an empty string will be used to
   + * denote the anonymous user.
   

   IMO, if this function is to be reused elsewhere, the comment should not make assumptions about how an empty, non-required, value is handled (ie empty string does not necessarily denote the anonymous user, it might just mean no user at all). Same comment for the comment within the function body.

2. +function user_element_validate_name_autocomplete($element) {
   

   The _autocomplete suffix feels a bit strange to me. Validation is not really related to the autocomplete portion of this element. What about user_validate_author($element)?

3. PageEditTestCase in node.test currently does not test author/username validation. Should this patch add it or is that outside of the scope of this patch?

does not apply anymore

This code has changed so much in the meantime that we don't even validate the comment user name like that anymore. Feel free to reopen if this issue still exists.