reintroduce calloc elison of memset for direct-mmapped allocations

a new weak predicate function replacable by the malloc implementation,
__malloc_allzerop, is introduced. by default it's always false; the
default version will be used when static linking if the bump allocator
was used (in which case performance doesn't matter) or if malloc was
replaced by the application. only if the real internal malloc is
linked (always the case with dynamic linking) does the real version
get used.

if malloc was replaced dynamically, as indicated by __malloc_replaced,
the predicate function is ignored and conditional-memset is always
performed.

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