Per previous discussions, get rid of use of sync(2) in favor of

explicitly fsync'ing every (non-temp) file we have written since the
last checkpoint.  In the vast majority of cases, the burden of the
fsyncs should fall on the bgwriter process not on backends.  (To this
end, we assume that an fsync issued by the bgwriter will force out
blocks written to the same file by other processes using other file
descriptors.  Anyone have a problem with that?)  This makes the world
safe for WIN32, which ain't even got sync(2), and really makes the world
safe for Unixen as well, because sync(2) never had the semantics we need:
it offers no way to wait for the requested I/O to finish.

Along the way, fix a bug I recently introduced in xlog recovery:
file truncation replay failed to clear bufmgr buffers for the dropped
blocks, which could result in 'PANIC:  heap_delete_redo: no block'
later on in xlog replay.

src/include/storage/bufmgr.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.80 2004/05/29 22:48:23 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.81 2004/05/31 03:48:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -154,7 +154,8 @@ extern BlockNumber RelationGetNumberOfBlocks(Relation relation);
 extern void RelationTruncate(Relation rel, BlockNumber nblocks);
 extern int	FlushRelationBuffers(Relation rel, BlockNumber firstDelBlock);
 extern void DropRelationBuffers(Relation rel);
-extern void DropRelFileNodeBuffers(RelFileNode rnode, bool istemp);
+extern void DropRelFileNodeBuffers(RelFileNode rnode, bool istemp,
+								   BlockNumber firstDelBlock);
 extern void DropBuffers(Oid dbid);
 
 #ifdef NOT_USED