Make the LLError::Settings LLSingleton duplicate the file handle for stderr
(usually 2) on construction. Make its destructor restore the original target
for that file handle. Provide a getDupStderr() method to obtain the duplicate
file handle.

Move Settings declaration up to the top of the file so other code can
reference it.

Make RecordToFile (the Recorder subclass engaged by LLError::logToFile()),
instead of duplicating stderr's file handle itself, capture the duplicate
stderr file handle from Settings to revert stderr redirection on destruction.

Make RecordToStderr (the Recorder subclass engaged by LLError::logToStderr())
use fdopen() to create an LLFILE* targeting the duplicate file handle from
Settings. Write output to that instead of to stderr so logToStderr() continues
to provide output for the user instead of duplicating each line into the log
file.