pgsql: Allow AS to be omitted when specifying an output column name in

Log Message:
-----------
Allow AS to be omitted when specifying an output column name in SELECT
(or RETURNING), but only when the output name is not any SQL keyword.
This seems as close as we can get to the standard's syntax without a
great deal of thrashing.  Original patch by Hiroshi Saito, amended by me.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        queries.sgml (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/queries.sgml?r1=1.44&r2=1.45)
        sql.sgml (r1.46 -> r1.47)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/sql.sgml?r1=1.46&r2=1.47)
    pgsql/doc/src/sgml/ref:
        delete.sgml (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/delete.sgml?r1=1.32&r2=1.33)
        insert.sgml (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/insert.sgml?r1=1.35&r2=1.36)
        select.sgml (r1.102 -> r1.103)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml?r1=1.102&r2=1.103)
        select_into.sgml (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select_into.sgml?r1=1.39&r2=1.40)
        update.sgml (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/update.sgml?r1=1.45&r2=1.46)
    pgsql/src/backend/parser:
        gram.y (r2.606 -> r2.607)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.606&r2=2.607)
    pgsql/src/interfaces/ecpg/preproc:
        preproc.y (r1.360 -> r1.361)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/preproc.y?r1=1.360&r2=1.361)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out?r1=1.58&r2=1.59)