modffi: Describe typecodes where they differ from used by struct module.

Exact behavior of typecodes may be not yet enforced.
modffi: Don't use OSError for clearly unrelated errors.
objexcept: Add mp_obj_new_exception_arg1() convenience function.
unix: OSError's args[0] should be errno numeric value.

Well, Python3 also defines an attribute for that, but that's bloat.
modsocket: Clean up OSError-like exception a bit.

Some BSD socket functions don't return error numbers in errno namespace, but
rather in other error namespaces. CPython resolves this by using OSError
subclasses for them. We don't do that so far, so there's ambiguity here.