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/src/tcpsockets/SocketError.cpp

@@ -0,0 +1,31 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "tcpsockets/SocketError.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <string.h>
+#endif
+
+namespace wpi {
+
+std::string SocketStrerror(int code) {
+#ifdef _WIN32
+  LPSTR errstr = nullptr;
+  FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
+                0, code, 0, (LPSTR)&errstr, 0, 0);
+  std::string rv(errstr);
+  LocalFree(errstr);
+  return rv;
+#else
+  return strerror(code);
+#endif
+}
+
+}  // namespace wpi