Patch for psql 8.0, 8.1 and 8.2 backwards compatibility

"Ugh, I started the wrong version of psql again".

This patch offers basic backwards compatibility, so a version 8.4 psql
can successfully do common operations on Postgres 8.0, 8.1, 8.2 and
8.3. I expect it's incomplete support, but as of yet I can't find an
actual problem. To me it is a step forward regardless, as it fixes "\d"
which is pretty crucial even when just popping to an old server to check
on something before an upgrade.

Notes:
1) Patched against cvs as of 19 May 2008.
2) The exact version numbers for the tweaks need help from a guru.
3) The usual version mismatch warning remains, as this is not a promise
of full compatibility.

Bryce Nesbitt

PS: See also:
http://archives.postgresql.org/pgsql-bugs/2005-08/msg00306.php
http://unixadmintalk.com/f47/improved-df-psql-backward-compatibility-345293/
http://archives.postgresql.org/pgsql-sql/2008-02/msg00088.php