net: add udp socket support and a simple udp server example.

vlib/net/socket_udp_test.v

@@ -0,0 +1,18 @@
+import net
+
+fn start_socket_udp_server() {
+	bufsize := 1024
+	bytes := [1024]byte
+	s := net.socket_udp() or { panic(err) }
+	_ := s.bind( 9876 ) or { panic(err) }
+	println('Waiting for udp packets:')
+	for {
+		res := s.crecv(bytes, bufsize)
+		if res < 0 { break }
+		print('Received $res bytes: ' + tos(bytes, res))
+	}
+}
+
+fn test_udp_server() {
+	// start_socket_udp_server()
+}