PITR Archival

I enclose a working set of context diff patches and new files to make
PITR archiving work, as of cvstip (NOW).

You'll see the new options in the postgresql.conf...though you may wish
to use archive_debug = true as well, when testing.

There is one bug: shutdown doesn't work quite right. I haven't fixed
this because I've spent too long trying to decipher how pgstat did a
clean shutdown, discovering now that it didn't and that has now been
patched...something similar is required for pgarch, but I'm out of time
now...leaving time for discussion of this lot...

I'm looking to have this lot committed asap, cos my fingers are starting
to catch the bitrot now. :)

I have a considerable amount still to learn about CVS, diff and patch,
so anybody wanting to spend 10-15 mins on the phone with me would
greatly enhance my chances of helping patch my patch, when the bugs roll
in.

If we get this done smoothly, I reckon I can have some PITR recovery
control done by beta freeze.

Best regards, Simon Riggs