Save errno before using sprintf, ifstream.

std::ifstream in libstdc++ contains a bug, where it sets errno to zero.
To work around it, we manually save the errno before using std::ifstream
in debugger check, and reset it after we are done.

We also preventively save errno before using sprintf.

Fixes #835

include/internal/catch_errno_guard.hpp

@@ -0,0 +1,25 @@
+/*
+ *  Created by Martin on 06/03/2017.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#ifndef TWOBLUECUBES_CATCH_ERRNO_GUARD_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_ERRNO_GUARD_HPP_INCLUDED
+
+#include <cerrno>
+
+
+namespace Catch {
+
+    class ErrnoGuard {
+    public:
+        ErrnoGuard():m_oldErrno(errno){}
+        ~ErrnoGuard() { errno = m_oldErrno; }
+    private:
+        int m_oldErrno;
+    };
+
+}
+
+#endif // TWOBLUECUBES_CATCH_ERRNO_GUARD_HPP_INCLUDED