Make pg_stop_backup's wait_for_archive flag work on standbys.

Previously, it had no effect.  Now, if archive_mode=always, it will
work, and if not, you'll get a warning.

Masahiko Sawada, Michael Paquier, and Robert Haas.  The patch as
submitted also changed the behavior so that we would write and remove
history files on standbys, but that seems like material for a separate
patch to me.

Discussion: http://postgr.es/m/CAD21AoC2Xw6M=ZJyejq_9d_iDkReC_=rpvQRw5QsyzKQdfYpkw@mail.gmail.com