Include WAL in base backup

Here's a cutdown version of the idea about including WAL in the base
backup. What I initially wanted was to introduce a way to guarantee
that the required WAL (with some sort of cutoff of course) would be
available for the backup, but I ran out of time for that. We can
always add that later.

For now, you need to set wal_keep_segments to make it work properly,
but if you do the idea is that the tar file/stream generated in the
base backup will include all the required WAL files. That means that
you can start a postmaster directly against the directory extracted
from the tarball, and you no longer need to set up archive logging to
get a backup.

I've got some refactoring I want to do around the
SendBackupDirectory() function after this, but a review of the
functionality first would be good. And obviously, documentation is
still necessary.

The patch to pg_basebackup applies on top of the previously posted version.

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/