fixed usage of flags in poll

Usage in production of version 3.2.0 has shown that the poll usage in the ThreadPoolServer under linux was not optimal as it was lacking the flag POLLRDHUP
I've thus added it (and fixed on the way the usage of the other flags).

To be honest, I do not like the way I wrote this code as I hardcoded the value of POLLRDHUP (0x2000). On the other hand, this constant is not available in python and when missing, we are getting accumulation of CLOSEWAIT sockets that finally kill our servers.
Feel free to propose a better fix :-)