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/test.sh.in

@@ -43,7 +43,6 @@ for t in $@; do
     # per test shell script to ignore the @JEMALLOC_CPREFIX@ detail).
     $(enable_fill=@enable_fill@ \
       enable_prof=@enable_prof@ \
-      enable_tcache=@enable_tcache@ \
       . @srcroot@${t}.sh && \
       export_malloc_conf && \
       ${t}@exe@ @abs_srcroot@ @abs_objroot@)