PAM-mapped user name saved to a wrong field

[Laurynas Biveinis]

The PAM stack may decide to map a given user name to some other user name. In that case pam_get_item(..., PAM_USER, ...) will return the mapped user name that is different from MYSQL_SERVER_AUTH_INFO::user_name.

The current code the copies that value to MYSQL_SERVER_AUTH_INFO::external_user field, which is incorrect. It shold copy it to the authenticated_as field, so that the mapped user name is checked against MySQL user table for privileges.