Use the original username in signup validation

[donncha]

During signup, the original username is lost if it contains any unusual characters because of the sanitize_user() call. This patch restores the username so the signup page shows the original username again. The signup form will display an error as well allowing the user to modify it.

The current behaviour returns a modified but acceptable username. Some users won't notice and create that user then get angry when they realise it has changed.

This patch also adds the original username to the $result array passed to the "wpmu_validate_user_signup" filter.

[donncha]

(In [14298]) Display the original username on the signup form on error, fixes #13124