Convert the arithmetic for shared memory size calculation from 'int'

to 'Size' (that is, size_t), and install overflow detection checks in it.
This allows us to remove the former arbitrary restrictions on NBuffers
etc.  It won't make any difference in a 32-bit machine, but in a 64-bit
machine you could theoretically have terabytes of shared buffers.
(How efficiently we could manage 'em remains to be seen.)  Similarly,
num_temp_buffers, work_mem, and maintenance_work_mem can be set above
2Gb on a 64-bit machine.  Original patch from Koichi Suzuki, additional
work by moi.

src/backend/access/transam/slru.c

@@ -48,7 +48,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.26 2005/07/04 04:51:44 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.27 2005/08/20 23:26:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -141,9 +141,10 @@ static int	SlruSelectLRUPage(SlruCtl ctl, int pageno);
  * Initialization of shared memory
  */
 
-int
+Size
 SimpleLruShmemSize(void)
 {
+	/* we assume NUM_SLRU_BUFFERS isn't so large as to risk overflow */
 	return BUFFERALIGN(sizeof(SlruSharedData)) + BLCKSZ * NUM_SLRU_BUFFERS;
 }