fix bug introduced by 7cef6c46552a988c1c710a0a2404316a27ef1d43
stack_start used to be stack_start + stack_size, so re-setting stk to stack_start would set the pointer to the *end* of the stack instead of the beginning.
add cunit
add sin6_scope_id to sockaddr6_t for compatibility (currently unused by RIOT)
add compatibility stubs