Review of last summer's PITR patch

I've done some preliminary work on the PITR patch that J.R. Nield
developed last summer. I've applied parts of it (in rather edited
form) but there are big chunks I think are not ready to go. You
can see the original patch in pgsql-patches --- I sent it there
mostly to get it into the archives.

I have committed the parts that have to do with making the contents of
WAL more robust for PITR purposes; specifically, labeling WAL segment
files clearly, and adding WAL logging of file creation/deletion.

I have not committed the massive reorganization of xlog.c that appears
in J.R.'s patch; I think it's unnecessary and likely to introduce bugs
(certainly it would revert some recent bug fixes).

I have also not committed the ALTER SYSTEM BACKUP / ALTER SYSTEM RECOVER
commands that appear in the patch. It is not clear to me that this
functionality belongs in the backend rather than separate management
utilities, and even if it does belong there, this doesn't seem a clean
way to do it. On the backup side, it seems like this code is basically
reinventing the tar(1) command. On the restore side, I don't care for
the "interactive command" implementation of ALTER SYSTEM RECOVER ---
that seems like it makes it unnecessarily hard to drive the recovery
process from another program. I envision this process being controlled
by some kind of higher-level management utility, so I'd prefer to see a
program-friendly API instead of one designed for manual use.

Anyway, I'm hoping to see some discussion of what to do next and what
the PITR functionality ought to look like from a user's standpoint.

regards, tom lane