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.

Modified Files:
--------------
    pgsql/src/backend/executor:
        spi.c (r1.197 -> r1.198)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.197&r2=1.198)
    pgsql/src/backend/utils/adt:
        ri_triggers.c (r1.109 -> r1.110)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ri_triggers.c?r1=1.109&r2=1.110)
    pgsql/src/backend/utils/cache:
        plancache.c (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/plancache.c?r1=1.21&r2=1.22)
    pgsql/src/include/executor:
        spi.h (r1.66 -> r1.67)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h?r1=1.66&r2=1.67)
    pgsql/src/include/utils:
        plancache.h (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/plancache.h?r1=1.13&r2=1.14)