DRTVWR-476, SL-13528: Use freopen_s() instead of assigning stderr.

The llappviewerwin32.cpp create_console() function called by
LLAppViewerWin32::initConsole() used to assign *stderr = *(new FILE* value),
and so forth for stdout and stdin. That dubious tactic no longer works with
the new Windows CRT introduced with VS 2015. freopen_s() works much better.