pgsql: Fix caching of foreign-key-checking queries so that when a replan

Log Message:
-----------
Fix caching of foreign-key-checking queries so that when a replan is needed,
we regenerate the SQL query text not merely the plan derived from it.  This
is needed to handle contingencies such as renaming of a table or column
used in an FK.  Pre-8.3, such cases worked despite the lack of replanning
(because the cached plan needn't actually change), so this is a regression.
Per bug #4417 from Benjamin Bihler.

Tags:
----
REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/executor:
        spi.c (r1.188.2.1 -> r1.188.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.188.2.1&r2=1.188.2.2)
    pgsql/src/backend/utils/adt:
        ri_triggers.c (r1.103.2.2 -> r1.103.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ri_triggers.c?r1=1.103.2.2&r2=1.103.2.3)
    pgsql/src/backend/utils/cache:
        plancache.c (r1.15 -> r1.15.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/plancache.c?r1=1.15&r2=1.15.2.1)
    pgsql/src/include/executor:
        spi.h (r1.65 -> r1.65.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h?r1=1.65&r2=1.65.2.1)
    pgsql/src/include/utils:
        plancache.h (r1.11 -> r1.11.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/plancache.h?r1=1.11&r2=1.11.2.1)