pgsql: Fix PREPARE TRANSACTION to reject the case where the transaction

Log Message:
-----------
Fix PREPARE TRANSACTION to reject the case where the transaction has dropped a
temporary table; we can't support that because there's no way to clean up the
source backend's internal state if the eventual COMMIT PREPARED is done by
another backend.  This was checked correctly in 8.1 but I broke it in 8.2 :-(.
Patch by Heikki Linnakangas, original trouble report by John Smith.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.249 -> r1.249.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.249&r2=1.249.2.1)
    pgsql/src/backend/access/transam:
        xact.c (r1.257 -> r1.257.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.257&r2=1.257.2.1)
    pgsql/src/backend/storage/lmgr:
        lmgr.c (r1.96 -> r1.96.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lmgr.c?r1=1.96&r2=1.96.2.1)
        lock.c (r1.181 -> r1.181.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c?r1=1.181&r2=1.181.2.1)
    pgsql/src/include/access:
        xact.h (r1.93 -> r1.93.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xact.h?r1=1.93&r2=1.93.2.1)
    pgsql/src/include/storage:
        lmgr.h (r1.60 -> r1.60.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lmgr.h?r1=1.60&r2=1.60.2.1)