Skip setting MIME boundaries when Mimemail module is available

When the mimemail module is enabled and mimemail's `drupal_mail_wrapper()` is in use, then skip setting our own MIME boundaries in `email_confirmation_mail()`. Mimemail will handle this itself.

Also, a change to how we set `Return-Path` and `Errors-To` headers to avoid a sendmail error when using mimemail: now using only the email address itself, and not the full `from_name` and `from_address` string.