Turn on -o filenames in _command_offset() for completions that specify it.

We don't need to do "complete -o filenames ..." when installing
bash-completion completions any longer (they should be handled
"internally"), but there are external completions which do that and
need it in effect also when completing with nice, sudo and friends.

bash_completion

@@ -1518,6 +1518,8 @@ _command_offset()
                     cspec=${cspec%% *}
                     if [[ "$cspec" != @(dir|file)names ]]; then
                         COMPREPLY=("${COMPREPLY[@]//\\\\:/:}")
+                    else
+                        _compopt_o_filenames
                     fi
                 fi
             elif [ -n "$cspec" ]; then