extmod/modlwip: Fix polling of UDP socket so it doesn't return HUP.

STATE_NEW will return HUP when polled so put active UDP sockets into a new
state which is different to STATE_NEW.

Fixes issue #5758.

tests/extmod/uselect_poll_udp.py

@@ -0,0 +1,28 @@
+# test select.poll on UDP sockets
+
+try:
+    import usocket as socket, uselect as select
+except ImportError:
+    try:
+        import socket, select
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+    
+
+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+s.bind(socket.getaddrinfo('127.0.0.1', 8000)[0][-1])
+poll = select.poll()
+
+# UDP socket should not be readable
+poll.register(s, select.POLLIN)
+print(len(poll.poll(0)))
+
+# UDP socket should be writable
+poll.modify(s, select.POLLOUT)
+print(poll.poll(0)[0][1] == select.POLLOUT)
+
+# same test for select.select, but just skip it if the function isn't available
+if hasattr(select, "select"):
+    r, w, e = select.select([s], [], [], 0)
+    assert not r and not w and not e