[Groff] output F command makes sense

[Bernd Warken]

Some days ago, it was agreed that the F command of groff_out was 
obsolete.  But it is actually used by groff within piping.  It ensures
that the filename isn't lost when the input is internally piped to
standard output.

So I restored it to libdriver/input.cc in the sense "set intended file 
name".  It, however, may not be used to change the input file as within
the processing. 

Bernd Warken