-HEAD pg_dumpall broken against older backends

Hi!

looks like somebody forgot to test some changes to the pg_dumpall code
in Revision 1.70 against <8.1 installations - resulting in the
following syntax error:

--
-- PostgreSQL database cluster dump
--

\connect postgres

pg_dumpall: query failed: ERROR: syntax error at or near "null"
LINE 1: ... passwd as rolpassword, valuntil as rolvaliduntil null as ro...
^
pg_dumpall: query was: SELECT usename as rolname, usesuper as rolsuper,
true as rolinherit, usesuper as rolcreaterole, usecreatedb as
rolcreatedb, usecatupd as rolcatupdate, true as rolcanlogin, -1 as
rolconnlimit, passwd as rolpassword, valuntil as rolvaliduntil null as
rolcomment FROM pg_shadow UNION ALL SELECT groname as rolname, false as
rolsuper, true as rolinherit, false as rolcreaterole, false as
rolcreatedb, false as rolcatupdate, false as rolcanlogin, -1 as
rolconnlimit, null::text as rolpassword, null::abstime as rolvaliduntil
null FROM pg_group

proposed patch to fix problem is attached.

Stefan