Fix a couple of problems pointed out by Fujii Masao in the 2008-Apr-05 patch

for pg_stop_backup.  First, it is possible that the history file name is not
alphabetically later than the last WAL file name, so we should explicitly
check that both have been archived.  Second, the previous coding would wait
forever if a checkpoint had managed to remove the WAL file before we look for
it.

Simon Riggs, plus some code cleanup by me.