ctypes: c_ssize_t introduced from 2.7

I do not know it's right or not
callback: fix arg type

cb arg type was mixed up c_void_p and py_object, fix using py_object

Could not get errno EINTR even NLM_F_DUMP_INTR flag set in Python2.6
I can not see where is wrong...
examples: make it working

but ENOBUFS may occur, seems to need buffer tuning