Updated version of pg_receivexlog

Here's an updated version of pg_receivexlog, that should now actually
work (it previously failed miserably when a replication record crossed
a WAL file boundary - something which I at the time could not properly
reproduce, but when I restarted my work on it now could easily
reproduce every time :D).

It also contains an update to pg_basebackup that allows it to stream
the transaction log in the background while the backup is running,
thus reducing the need for wal_keep_segments (if the client can keep
up, it should eliminate the need completely).

In doing so, it moves a number of functions from pg_basebackup.c to
the new file streamutil.c, to be shared between both pg_basebackup and
pg_receivexlog.

So far at least, it's completely client-side, with no changes to the
server. This means that it can be dropped into src/bin on 9.1 as well
to get a version that runs there (since we're way way way past feature
freeze and can't actually stick it in there in the official tree)

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