memchecker: per RFC, use calloc for OBJ_NEW

I think that this is worthy of bringing to v1.8 because it will significantly reduce valgrind output for those who do ```--enable-memchecker``` builds.  Hence, it can easily be considered a bug fix, not an enhancement.

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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.

(cherry picked from commit a422d893b8e63347deb71bbaaa14ca25ddb1f192)