Remove --disable-tcache.

Simplify configuration by removing the --disable-tcache option, but
replace the testing for that configuration with
--with-malloc-conf=tcache:false.

Fix the thread.arena and thread.tcache.flush mallctls to work correctly
if tcache is disabled.

This partially resolves #580.

test/unit/decay.sh

@@ -1,6 +1,3 @@
 #!/bin/sh
 
-export MALLOC_CONF="dirty_decay_time:1,muzzy_decay_time:1"
-if [ "x${enable_tcache}" = "x1" ] ; then
-  export MALLOC_CONF="${MALLOC_CONF},lg_tcache_max:0"
-fi
+export MALLOC_CONF="dirty_decay_time:1,muzzy_decay_time:1,lg_tcache_max:0"