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.

INSTALL

@@ -153,11 +153,6 @@ any of the following arguments (not a definitive list) to 'configure':
     Statically link against the specified libunwind.a rather than dynamically
     linking with -lunwind.
 
---disable-tcache
-    Disable thread-specific caches for small objects.  Objects are cached and
-    released in bulk, thus reducing the total number of mutex operations.  See
-    the "opt.tcache" option for usage details.
-
 --disable-munmap
     Disable virtual memory deallocation via munmap(2); instead keep track of
     the virtual memory for later use.  munmap() is disabled by default (i.e.