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/java/edu/wpi/first/wpiutil/WPIUtilJNI.java

@@ -0,0 +1,56 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpiutil;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+public final class WPIUtilJNI {
+  static boolean libraryLoaded = false;
+  static RuntimeLoader<WPIUtilJNI> loader = null;
+
+  public static class Helper {
+    private static AtomicBoolean extractOnStaticLoad = new AtomicBoolean(true);
+
+    public static boolean getExtractOnStaticLoad() {
+      return extractOnStaticLoad.get();
+    }
+
+    public static void setExtractOnStaticLoad(boolean load) {
+      extractOnStaticLoad.set(load);
+    }
+  }
+
+  static {
+    if (Helper.getExtractOnStaticLoad()) {
+      try {
+        loader = new RuntimeLoader<>("wpiutiljni", RuntimeLoader.getDefaultExtractionRoot(), WPIUtilJNI.class);
+        loader.loadLibrary();
+      } catch (IOException ex) {
+        ex.printStackTrace();
+        System.exit(1);
+      }
+      libraryLoaded = true;
+    }
+  }
+
+  /**
+   * Force load the library.
+   */
+  public static synchronized void forceLoad() throws IOException {
+    if (libraryLoaded) {
+      return;
+    }
+    loader = new RuntimeLoader<>("wpiutiljni", RuntimeLoader.getDefaultExtractionRoot(), WPIUtilJNI.class);
+    loader.loadLibrary();
+    libraryLoaded = true;
+  }
+
+  public static native void addPortForwarder(int port, String remoteHost, int remotePort);
+  public static native void removePortForwarder(int port);
+}

wpiutil/src/main/java/edu/wpi/first/wpiutil/net/PortForwarder.java

@@ -0,0 +1,39 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpiutil;
+
+/**
+ * Forward ports to another host.  This is primarily useful for accessing
+ * Ethernet-connected devices from a computer tethered to the RoboRIO USB port.
+ */
+public final class PortForwarder {
+  private PortForwarder() {
+    throw new UnsupportedOperationException("This is a utility class!");
+  }
+
+  /**
+   * 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
+   */
+  public static void add(int port, String remoteHost, int remotePort) {
+    WPIUtilJNI.addPortForwarder(port, remoteHost, remotePort);
+  }
+
+  /**
+   * Stop TCP forwarding on a port.
+   *
+   * @param port local port number
+   */
+  public static void remove(int port) {
+    WPIUtilJNI.removePortForwarder(port);
+  }
+}