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.

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git-svn-id: http://core.svn.wordpress.org/trunk@28274 1a063a9b-81f0-0310-95a4-ce76da25c4cd