Sensible default when converting between types with common fields

A field can be shared between multiple content types. When a node is converted to another content type that has some of the same fields, it should use the source field as the destination field by default.

The attached patch accomplishes this by setting the source field name as the default value for the destination field select list. If the source field is not available as an option, Drupal ignores it and behaves as though no default were set.