[RFA] use filename_completer for add_setshow_optional_filename_cmd

[Joel Brobecker]

> There are currently only three adffected commands:
> 
> "set debug-file-directory"
> "set args"
> "set solib-search-path"
> 
> Only the second one might be questionable, but I also find that it
> is reasonable to have filename completion for "set args" as this is
> now done on most shells.

I also agree that this is more useful than completing on symbols.

> 2008-07-01  Pierre Muller  <muller@ics.u-strasbg.fr>
> 
> 	cli/cli-decode.c (add_setshow_optional_filename_cmd): Set
> 	completer for set to filename_completer.

This change makes sense. Can you submit a NEWS entry describing the
new completion policy? The code update itself is approved.

Thank you,
-- 
Joel