pgsql: Arrange for PreventTransactionChain to reject commands submitted

Log Message:
-----------
Arrange for PreventTransactionChain to reject commands submitted as part
of a multi-statement simple-Query message.  This bug goes all the way
back, but unfortunately is not nearly so easy to fix in existing releases;
it is only the recent ProcessUtility API change that makes it fixable in
HEAD.  Per report from William Garrison.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.237 -> r1.238)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.237&r2=1.238)
    pgsql/src/backend/tcop:
        postgres.c (r1.528 -> r1.529)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.528&r2=1.529)