Add utility classes (Slice, Fd, Status, etc)

Test: as follows
    - built
    - flashed
    - bootedt log
    - netdutils_test passes
    - unsubmitted NFLogListenerTest passes
Bug: 28806131

Change-Id: I17846a0dee1edca55df10e5464e607109d978cb5
diff --git a/libnetdutils/include/netdutils/Fd.h b/libnetdutils/include/netdutils/Fd.h
new file mode 100644
index 0000000..7db4087
--- /dev/null
+++ b/libnetdutils/include/netdutils/Fd.h
@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef NETUTILS_FD_H
+#define NETUTILS_FD_H
+
+#include <ostream>
+
+#include "netdutils/Status.h"
+
+namespace android {
+namespace netdutils {
+
+// Strongly typed wrapper for file descriptors with value semantics.
+// This class should typically hold unowned file descriptors.
+class Fd {
+  public:
+    constexpr Fd() = default;
+
+    constexpr Fd(int fd) : mFd(fd) {}
+
+    int get() const { return mFd; }
+
+    bool operator==(const Fd& other) const { return get() == other.get(); }
+    bool operator!=(const Fd& other) const { return get() != other.get(); }
+
+  private:
+    int mFd = -1;
+};
+
+// Return true if fd appears valid (non-negative)
+inline bool isWellFormed(const Fd fd) {
+    return fd.get() >= 0;
+}
+
+std::ostream& operator<<(std::ostream& os, const Fd& fd);
+
+}  // namespace netdutils
+}  // namespace android
+
+#endif /* NETUTILS_FD_H */