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.

doc/zmq_poll.txt

@@ -69,6 +69,13 @@ condition is present on the socket specified by 'fd'. For 0MQ sockets this flag
 has no effect if set in 'events', and shall never be returned in 'revents' by
 _zmq_poll()_.
 
+*ZMQ_POLLPRI*::
+For 0MQ sockets this flags is of no use. For standard sockets this means there
+is urgent data to read. Refer to the POLLPRI flag for more informations.
+For file descriptor, refer to your use case: as an example, GPIO interrupts
+are signaled through a POLLPRI event.
+This flag has no effect on Windows.
+
 NOTE: The _zmq_poll()_ function may be implemented or emulated using operating
 system interfaces other than _poll()_, and as such may be subject to the limits
 of those interfaces in ways not defined in this documentation.

include/zmq.h

@@ -373,6 +373,7 @@ ZMQ_EXPORT int zmq_socket_monitor (void *s, const char *addr, int events);
 #define ZMQ_POLLIN 1
 #define ZMQ_POLLOUT 2
 #define ZMQ_POLLERR 4
+#define ZMQ_POLLPRI 8
 
 typedef struct zmq_pollitem_t
 {

src/zmq.cpp

@@ -722,7 +722,8 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
             pollfds [i].fd = items_ [i].fd;
             pollfds [i].events =
                 (items_ [i].events & ZMQ_POLLIN ? POLLIN : 0) |
-                (items_ [i].events & ZMQ_POLLOUT ? POLLOUT : 0);
+                (items_ [i].events & ZMQ_POLLOUT ? POLLOUT : 0) |
+                (items_ [i].events & ZMQ_POLLPRI ? POLLPRI : 0);
         }
     }
 
@@ -781,7 +782,9 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
                     items_ [i].revents |= ZMQ_POLLIN;
                 if (pollfds [i].revents & POLLOUT)
                     items_ [i].revents |= ZMQ_POLLOUT;
-                if (pollfds [i].revents & ~(POLLIN | POLLOUT))
+                if (pollfds [i].revents & POLLPRI)
+                   items_ [i].revents |= ZMQ_POLLPRI;
+                if (pollfds [i].revents & ~(POLLIN | POLLOUT | POLLPRI))
                     items_ [i].revents |= ZMQ_POLLERR;
             }