Eliminate use of `extract()` in `validate_blog_signup()`:

* `$orig_username` does not need to be pulled from `wpmu_validate_user_signup()` as it is not used in this function.
* `$user` does not need to be pulled from `wpmu_validate_blog_signup()` as it is not used in this function.
* For the `wpmu_validate_user_signup()` portion, rename `$result` and `$errors` to $user_result` and `$user_errors` for disambiguation with the blog values below.

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28447 602fd350-edb4-49c9-b593-d223f7449a82