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).
This flag does nothing if poll() is not the underlying polling
function. So it is Linux only.
Merge pull request #1216 from xaqq/pollpri

Add support for POLLPRI flag.