Disable full_page_writes, because turning it off risks causing crash-recovery

failures even when the hardware and OS did nothing wrong.  Per recent analysis
of a problem report from Alex Bahdushka.

For the moment I've just diked out the test of the parameter, rather than
removing the GUC infrastructure and documentation, in case we conclude that
there's something salvageable there.  There seems no chance of it being
resurrected in the 8.1 branch though.

src/backend/access/transam/xlog.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.228 2006/03/24 04:32:13 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.229 2006/03/28 22:01:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -980,8 +980,7 @@ XLogCheckBuffer(XLogRecData *rdata,
 	 */
 	*lsn = page->pd_lsn;
 
-	if (fullPageWrites &&
-		XLByteLE(page->pd_lsn, RedoRecPtr))
+	if (XLByteLE(page->pd_lsn, RedoRecPtr))
 	{
 		/*
 		 * The page needs to be backed up, so set up *bkpb