pgsql: Fix the torn-page hazard for PITR base backups by forcing full

Log Message:
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Fix the torn-page hazard for PITR base backups by forcing full page writes
to occur between pg_start_backup() and pg_stop_backup(), even if the GUC
setting full_page_writes is OFF.  Per discussion, doing this in combination
with the already-existing checkpoint during pg_start_backup() should ensure
safety against partial page updates being included in the backup.  We do
not have to force full page writes to occur during normal PITR operation,
as I had first feared.

Modified Files:
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    pgsql/src/backend/access/transam:
        xlog.c (r1.235 -> r1.236)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.235&r2=1.236)