Add support for POLLPRI flag.

This commit adds a ZMQ_POLLPRI flag that maps to poll()'s POLLPRI
flag.
This flags does nothing for OMQ sockets. It's only useful for raw
file descriptor (be it socket or file).

I tested only on Linux. I think this does nothing on Windows because `poll()` is not the underlying polling function. I don't know if there is an windows equivalent for this, I therefore stated that this flags does nothing on Windows.