Minor recovery changes

As discussed on -hackers and -admin... a few minor changes, rolled into
a single patch.

- check we have an archiver to carry away WAL files at shutdown
(was part of task 1)

- archive_mode GUC, on which archive_command is dependent
closing loophole to avoid data loss with wal-avoiding commands
(was task 10)

- %r option in recovery.conf sends last restartpoint to recovery command
(was task 3)

- %r handled in pg_standby, updated README
(was task 4)

- code cleanup of pg_standby in various minor places

- doc on Warm Standby now includes pg_standby and %r

- log_restartpoints shows LOG message for at each restartpoint, rather
than DEBUG2 e.g.
LOG:  recovery restart point at 0/42000020
(was task 5)

- end of recovery now displays last transaction end time, as requested
by Warren Little     e.g.
LOG:  last completed transaction was at log time 2007-04-08 18:07:11 BST
also shown at each restartpoint
(discussed on -admin, mar 30)

TODO (by me)

- docs need another thorough refresh to pick up minor mentions of
archive_command -> archive_mode

- work out how to keep archiver alive long enough to archive last WAL
file after a shutdown checkpoint... trickier than I imagined since it
overlaps need for fast exit when performing restart, per last edit of
the archiver code.

- agree whether to allow pg_stop_backup() to exit before file archived

Applies cleanly to CVS HEAD. Updated test files, all options tested.

--
  Simon Riggs
  EnterpriseDB   http://www.enterprisedb.com