Add PortForwarder class (#1890)

This class provides an easy way to forward local ports to another host/port.
This is primarily useful to provide a way to access Ethernet-connected devices
from a computer tethered to the RoboRIO USB port.

The most natural spot to put the shared implementation of this class was into
wpiutil, so a wpiutilJNI library has been added.

wpiutil/src/main/native/include/wpi/PortForwarder.h

@@ -0,0 +1,62 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_WPI_PORTFORWARDER_H_
+#define WPIUTIL_WPI_PORTFORWARDER_H_
+
+#pragma once
+
+#include <memory>
+
+#include "wpi/Twine.h"
+
+namespace wpi {
+
+/**
+ * Forward ports to another host.  This is primarily useful for accessing
+ * Ethernet-connected devices from a computer tethered to the RoboRIO USB port.
+ */
+class PortForwarder {
+ public:
+  PortForwarder(const PortForwarder&) = delete;
+  PortForwarder& operator=(const PortForwarder&) = delete;
+
+  /**
+   * Get an instance of the PortForwarder class.
+   *
+   * This is a singleton to guarantee that there is only a single instance
+   * regardless of how many times GetInstance is called.
+   */
+  static PortForwarder& GetInstance();
+
+  /**
+   * Forward a local TCP port to a remote host and port.
+   * Note that local ports less than 1024 won't work as a normal user.
+   *
+   * @param port       local port number
+   * @param remoteHost remote IP address / DNS name
+   * @param remotePort remote port number
+   */
+  void Add(unsigned int port, const Twine& remoteHost, unsigned int remotePort);
+
+  /**
+   * Stop TCP forwarding on a port.
+   *
+   * @param port local port number
+   */
+  void Remove(unsigned int port);
+
+ private:
+  PortForwarder();
+
+  struct Impl;
+  std::unique_ptr<Impl> m_impl;
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_PORTFORWARDER_H_