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

CMakeLists.txt

@@ -135,6 +135,7 @@ set(INTERNAL_HEADERS
         ${HEADER_DIR}/internal/catch_debugger.h
         ${HEADER_DIR}/internal/catch_debugger.hpp
         ${HEADER_DIR}/internal/catch_default_main.hpp
+        ${HEADER_DIR}/internal/catch_errno_guard.hpp
         ${HEADER_DIR}/internal/catch_evaluate.hpp
         ${HEADER_DIR}/internal/catch_exception_translator_registry.hpp
         ${HEADER_DIR}/internal/catch_expression_lhs.hpp