MINOR: tools: implement ha_aligned_zalloc()

MINOR: tools: implement ha_aligned_zalloc()

This one is exactly ha_aligned_alloc() followed by a memset(0), as
it will be convenient for a number of call places as a replacement
for calloc().

Note that ideally we should also have a calloc version that performs
basic multiply overflow checks, but these are essentially used with
numbers of threads times small structs so that's fine, and we already
do the same everywhere in malloc() calls.