Fall back to filename completion if no completer is defined.

If a gsu application does not implement subcommand completers, it's
best to  complete on file names, if only to make completion work in
the common case of redirections. For example we like to complete the
last word of a partial command line of the form

	app subcommand > /dev/nu

This did not work so far. With the patch applied, it does.

The application can always override the behaviour by implementing a
suitable completer.