WITH CHECK OPTION for auto-updatable views

Here's a rebased version of the patch implementing WITH CHECK OPTION
for auto-updatable views.

It now includes documentation, and a clearer description of the
patch's limitations --- WITH CHECK OPTION is only supported on
auto-updatable views, not trigger-updatable or rule-updatable views. I
believe that's compatible with the following features from the SQL
standard:

F311-04    Schema definition statement    CREATE VIEW: WITH CHECK OPTION
F751       View CHECK enhancements

Regards,
Dean