pgsql: Fix EXPLAIN and EXECUTE commands to pass portal parameters

Log Message:
-----------
Fix EXPLAIN and EXECUTE commands to pass portal parameters through to
the executor.  This allows, for example, JDBC clients to use '?' bound
parameters in these commands.  Per gripe from Virag Saksena.

Modified Files:
--------------
    pgsql/src/backend/commands:
        explain.c (r1.141 -> r1.142)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.141&r2=1.142)
        prepare.c (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.42&r2=1.43)
    pgsql/src/backend/tcop:
        utility.c (r1.249 -> r1.250)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.249&r2=1.250)
    pgsql/src/include/commands:
        explain.h (r1.24 -> r1.25)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/explain.h.diff?r1=1.24&r2=1.25)
        prepare.h (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/prepare.h.diff?r1=1.14&r2=1.15)