Update: Move common fl_fss functions to f_fss where possible, also add private-fss...

Update: Move common fl_fss functions to f_fss where possible, also add private-fss.{c,h}.

Some of the fl_fss functions can be moved to f_fss, now that f_utf is considered an exception-case core library.

Some of the fl_fss functions are being used within itself.
Use private functions to do this.