Move common utility classes to wpiutil library. (#79)

This is a breaking change to dependencies that use the static ntcore
library.  Unless the wpiutil library is also linked, linker errors will
result.  This does not affect the shared ntcore library.

wpiutil/include/tcpsockets/SocketError.h

@@ -0,0 +1,37 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. 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_TCPSOCKETS_SOCKETERROR_H_
+#define WPIUTIL_TCPSOCKETS_SOCKETERROR_H_
+
+#include <string>
+
+#ifdef _WIN32
+#include <WinSock2.h>
+#else
+#include <errno.h>
+#endif
+
+namespace wpi {
+
+static inline int SocketErrno() {
+#ifdef _WIN32
+  return WSAGetLastError();
+#else
+  return errno;
+#endif
+}
+
+std::string SocketStrerror(int code);
+
+static inline std::string SocketStrerror() {
+  return SocketStrerror(SocketErrno());
+}
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_TCPSOCKETS_SOCKETERROR_H_