Allow Drupal user names with "@"

Drupal 7 allows "@" in user usernames, so why does CiviCRM implement a set of different rules?

Drupal 7 does appear to have a field message that is inconsistent with what it actually does (see https://www.drupal.org/node/827468), but that's a different issue.

In all events, we really should 1) replicate Drupal rules as they are, or even better, 2) defer validation to Drupal and not replicate.