Improve shutdown during online backup, take 2

This patch replaces the previous version
http://archives.postgresql.org/pgsql-patches/2008-04/msg00004.php

As suggested by Simon Riggs in
http://archives.postgresql.org/pgsql-patches/2008-04/msg00013.php ,
a smart shutdown will now wait for online backup mode to finish.
The functions that touch "backup_label" have been moved to xlog.c.

As suggested by Heikki Linnakangas in
http://archives.postgresql.org/pgsql-patches/2008-04/msg00180.php ,
I have introduced a new postmaster state PM_WAIT_BACKUP.
In this state the postmaster will still accept connections.

Thanks for the feedback!
I'll try to add a link at the Wiki page.

Yours,
Laurenz Albe