attempt to fix the dontwait tests on linux. don't define sndmsg/recvmsg on windows

gevent/_socket3.py

@@ -322,6 +322,9 @@ class socket(object):
                     raise
             self._wait(self._read_event)
 
+    if hasattr(_socket.socket, 'sendmsg'):
+        # Only on Unix
+
         def recvmsg(self, *args):
             while True:
                 try:
@@ -413,15 +416,22 @@ class socket(object):
                     return 0
                 raise
 
-    def sendmsg(self, *args):
+    if hasattr(_socket.socket, 'sendmsg'):
+        # Only on Unix
+        def sendmsg(self, buffers, ancdata=(), flags=0, address=None):
             try:
-            return _socket.socket.sendmsg(self._sock, *args)
+                return _socket.socket.sendmsg(self._sock, buffers, ancdata, flags, address)
             except error as ex:
+                if flags & getattr(_socket, 'MSG_DONTWAIT', 0):
+                    # Enable non-blocking behaviour
+                    # XXX: Do all platforms that have sendmsg have MSG_DONTWAIT?
+                    raise
+
                 if ex.args[0] != EWOULDBLOCK or self.timeout == 0.0:
                     raise
                 self._wait(self._write_event)
                 try:
-                return _socket.socket.sendmsg(self._sock, *args)
+                    return _socket.socket.sendmsg(self._sock, buffers, ancdata, flags, address)
                 except error as ex2:
                     if ex2.args[0] == EWOULDBLOCK:
                         return 0