python - be: Limit Postgres.Query to 1 command.

Log Message:
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Limit Postgres.Query to 1 command. Similar to prepared statements('P'),
explicitly limit the Query to one command. Handling multiple commands meant the
allowance of returning multiple Portals(nasty). Also, prohibit Transaction
statements as the user is expected to use Postgres.Transact(hint about it too).
Change q_rpt to a Node, as only one RPT in the generated list is allowed,
so no need to store rpt list.
Other miscellaneous changes.

Modified Files:
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    be/include/pypg:
        query.h (r1.5 -> r1.6)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/query.h.diff?r1=1.5&r2=1.6)
    be/src:
        query.c (r1.10 -> r1.11)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/query.c.diff?r1=1.10&r2=1.11)