Increase upper bound for bgwriter_lru_maxpages.

There is no particularly good reason to limit this value to 1000,
so increase the limit to INT_MAX / 2, the same limit we use for
shared_buffers.  It's not clear how much practical effect larger
settings will have, but there seems no harm in letting people try it.

Jim Nasby, less a comment change I stripped out.

Discussion: http://postgr.es/m/f6e58a22-030b-eb8a-5457-f62fb08d701c@BlueTreble.com

src/backend/utils/misc/guc.c

@@ -2419,7 +2419,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&bgwriter_lru_maxpages,
-		100, 0, 1000,
+		100, 0, INT_MAX / 2, /* Same upper limit as shared_buffers */
 		NULL, NULL, NULL
 	},