New UserDB option for read-only attribute

* Identifies fields for which the session value will not overwrite
  the database value. Useful for protected fields that are only to
  be managed by site administrators, particularly for outboard fields
  where you don't want [userdb] to try to create an outboard record
  at all.

* Syntax similar to UserDB 'scratch' attribute, but only valid for keys:

  UserDb default  read_only  'field1, field2, field3'