pgsql: Prevent pg_upgrade from migrating databases that use reg* data

Log Message:
-----------
Prevent pg_upgrade from migrating databases that use reg* data types
where the oid is not preserved by pg_upgrade (everything but pg_type).
Update documentation.

Per bug report from depstein@alliedtesting.com.

Modified Files:
--------------
    pgsql/contrib/pg_upgrade:
        check.c (r1.12 -> r1.13)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/check.c?r1=1.12&r2=1.13)
        function.c (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/function.c?r1=1.6&r2=1.7)
        version_old_8_3.c (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/version_old_8_3.c?r1=1.6&r2=1.7)
    pgsql/doc/src/sgml:
        pgupgrade.sgml (r1.12 -> r1.13)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgupgrade.sgml?r1=1.12&r2=1.13)