#6 PyObject_AsReadBuffer api changed leading to segfaults in ip_cksum_add

[Daniel]

PyObject_AsReadBuffer api changed to use Py_ssize_t instead of int for length argument. ignoring this leads to segfaults, using the ip_cksum_add method. this patch fixes the python api use in dnet.pyx as well as replacing the 'type' argument names with 'type_' as the first is now prohibited in pyrex. a newly generated dnet.c is also included.