libc/nmalloc: Handle size_t overflows in posix_memalign().

libc/nmalloc: Handle size_t overflows in posix_memalign().

Some algorithms/programs try to heuristically deduce alignment rules.
Instead of confusing them with minimal allocations for size == (size_t)-7 and
friends at different alignment values, just return ENOMEM since POSIX does not
explictly specify how alignment overflows should behave. Programs should keep
track of pointers they try to allocate (including size == 0 case too).