use memzero(foo, length); for all memset(foo, 0, length); calls

In trying to track down a stupid linker bug, I noticed a bunch of
memset() calls that should be using memzero() to make it more "obvious"
that the options are correct (i.e. 0 is not the length, but the data to
set).  So fix up all current calls to memset(foo, 0, length) to
memzero(foo, length).