Fix incorrect archive truncation point calculation in the %r recovery_command

parameter. This fixes bug 4137 reported by Wojciech Strzalka, where a WAL
file is deleted too early when starting the recovery of a warm standby server.

Also add a sanity check in pg_standby so that it will refuse to delete anything
earlier than the file being restored, and improve the debug message in case
nothing is deleted.

Simon Riggs. Backpatch to 8.3, which is where %r was introduced.