Forwarded PITR patch

I'm attaching the end-result patch that I got from Patrick Macdonald
after he'd worked on J.R. Nield's PITR patch of last summer. This is
*not* for application --- it's nowhere near applying cleanly to CVS
tip anyway. I just want to get it into the PG mail archives.

For discussion please see the post I'm about to make in
pgsql-hackers-pitr.

regards, tom lane