Allow an alias to be attached directly to a JOIN ... USING

A small new feature in SQL:2016 allows attaching a table alias to a
JOIN/USING construct:

    <named columns join> ::=
      USING <left paren> <join column list> <right paren>
      [ AS <join correlation name> ]

(The part in brackets is new.)

This seems quite useful, and it seems the code would already support
this if we allow the grammar to accept this syntax.

Patch attached.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services