Make it possibly to specify GUC params per user and per database.

Create a new catalog pg_db_role_setting where they are now stored, and better
encapsulate the code that deals with settings into its realm.  The old
datconfig and rolconfig columns are removed.

psql has gained a \drds command to display the settings.

Backwards compatibility warning: while the backwards-compatible system views
still have the config columns, they no longer completely represent the
configuration for a user or database.

Catalog version bumped.