Allow opt.tcache_max to accept small size classes.

Previously all the small size classes were cached.  However this has downsides
-- particularly when page size is greater than 4K (e.g. iOS), which will result
in much higher SMALL_MAXCLASS.

This change allows tcache_max to be set to lower values, to better control
resources taken by tcache.

test/unit/tcache_max.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+export MALLOC_CONF="tcache_max:1024"