pgsql: Install a data-type-based solution for protecting pg_get_expr().

Log Message:
-----------
Install a data-type-based solution for protecting pg_get_expr().

Since the code underlying pg_get_expr() is not secure against malformed
input, and can't practically be made so, we need to prevent miscreants
from feeding arbitrary data to it. We can do this securely by declaring
pg_get_expr() to take a new datatype "pg_node_tree" and declaring the
system catalog columns that hold nodeToString output to be of that type.
There is no way at SQL level to create a non-null value of type pg_node_tree.
Since the backend-internal operations that fill those catalog columns
operate below the SQL level, they are oblivious to the datatype relabeling
and don't need any changes.

Modified Files:
--------------
pgsql/doc/src/sgml:
catalogs.sgml (r2.227 -> r2.228)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.227&r2=2.228)
func.sgml (r1.531 -> r1.532)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.531&r2=1.532)
pgsql/src/backend/bootstrap:
bootstrap.c (r1.261 -> r1.262)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/bootstrap/bootstrap.c?r1=1.261&r2=1.262)
pgsql/src/backend/utils/adt:
pseudotypes.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pseudotypes.c?r1=1.23&r2=1.24)
pgsql/src/include/catalog:
catversion.h (r1.597 -> r1.598)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.597&r2=1.598)
genbki.h (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/genbki.h?r1=1.6&r2=1.7)
pg_attrdef.h (r1.26 -> r1.27)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_attrdef.h?r1=1.26&r2=1.27)
pg_cast.h (r1.45 -> r1.46)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_cast.h?r1=1.45&r2=1.46)
pg_constraint.h (r1.41 -> r1.42)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_constraint.h?r1=1.41&r2=1.42)
pg_index.h (r1.50 -> r1.51)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_index.h?r1=1.50&r2=1.51)
pg_proc.h (r1.581 -> r1.582)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.581&r2=1.582)
pg_rewrite.h (r1.35 -> r1.36)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_rewrite.h?r1=1.35&r2=1.36)
pg_trigger.h (r1.39 -> r1.40)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_trigger.h?r1=1.39&r2=1.40)
pg_type.h (r1.212 -> r1.213)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_type.h?r1=1.212&r2=1.213)
pgsql/src/include/utils:
builtins.h (r1.355 -> r1.356)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.355&r2=1.356)
pgsql/src/test/regress/expected:
opr_sanity.out (r1.91 -> r1.92)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/opr_sanity.out?r1=1.91&r2=1.92)
type_sanity.out (r1.34 -> r1.35)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/type_sanity.out?r1=1.34&r2=1.35)
pgsql/src/test/regress/sql:
opr_sanity.sql (r1.74 -> r1.75)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/opr_sanity.sql?r1=1.74&r2=1.75)
type_sanity.sql (r1.34 -> r1.35)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/type_sanity.sql?r1=1.34&r2=1.35)