DRTVWR-476: Try to log stderr output from classic-C libraries.

Some of the libraries we use produce log output to stderr. Such output can be
informative, but is invisible unless you launch the viewer from a console. In
particular, it's invisible to anyone trying to diagnose a problem by reading
someone else's SecondLife.log file.

Make RecordToFile -- the Recorder subclass engaged by LLError::logToFile() --
redirect STDERR_FILENO to the newly-opened log file so that any subsequent
writes to stderr (or cerr, for that matter) will be captured in the log file.
But first duplicate the original stderr file handle, and restore it when
RecordToFile is destroyed. That way, output written to stderr during the final
moments of application shutdown should still appear on (console) stderr.