memchecker: per RFC, use calloc for OBJ_NEW

With --enable-memchecker builds, use calloc(3) for OBJ_NEW instead of
malloc(3).  This cuts down on a lot of valgrind/memory checker false
positive output.

Also make a minor change in the valgrind configure.m4; have it assign
0xf to a char.  The prior assignment (of 0xff) was warning about an
overflow.  This didn't really matter, but we might as well make the
test not have a gratuitious warning in it.