pgsql: Add support for an optional INTO clause to PL/PgSQL's EXECUTE

Log Message:
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Add support for an optional INTO clause to PL/PgSQL's EXECUTE command.
This allows the result of executing a SELECT to be assigned to a row
variable, record variable, or list of scalars. Docs and regression tests
updated. Per Pavel Stehule, improvements and cleanup by Neil Conway.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.69 -> r1.70)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.69&r2=1.70)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.72&r2=1.73)
        pl_exec.c (r1.141 -> r1.142)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.141&r2=1.142)
        plpgsql.h (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.60&r2=1.61)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.31&r2=1.32)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.26&r2=1.27)