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/cpp/jni/WPIUtilJNI.cpp

@@ -0,0 +1,54 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include <jni.h>
+
+#include "edu_wpi_first_wpiutil_WPIUtilJNI.h"
+#include "wpi/PortForwarder.h"
+#include "wpi/jni_util.h"
+
+using namespace wpi::java;
+
+extern "C" {
+
+JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved) {
+  JNIEnv* env;
+  if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK)
+    return JNI_ERR;
+
+  return JNI_VERSION_1_6;
+}
+
+JNIEXPORT void JNICALL JNI_OnUnload(JavaVM* vm, void* reserved) {}
+
+/*
+ * Class:     edu_wpi_first_wpiutil_WPIUtilJNI
+ * Method:    addPortForwarder
+ * Signature: (ILjava/lang/String;I)V
+ */
+JNIEXPORT void JNICALL
+Java_edu_wpi_first_wpiutil_WPIUtilJNI_addPortForwarder
+  (JNIEnv* env, jclass, jint port, jstring remoteHost, jint remotePort)
+{
+  wpi::PortForwarder::GetInstance().Add(static_cast<unsigned int>(port),
+                                        JStringRef{env, remoteHost}.str(),
+                                        static_cast<unsigned int>(remotePort));
+}
+
+/*
+ * Class:     edu_wpi_first_wpiutil_WPIUtilJNI
+ * Method:    removePortForwarder
+ * Signature: (I)V
+ */
+JNIEXPORT void JNICALL
+Java_edu_wpi_first_wpiutil_WPIUtilJNI_removePortForwarder
+  (JNIEnv* env, jclass, jint port)
+{
+  wpi::PortForwarder::GetInstance().Remove(port);
+}
+
+}  // extern "C"