arg: Add no_arg and get_arg helper functions.
The no_arg function in this patch is a no-op. It will do something
useful in the followups.
The get_arg function simply checks the next position on the command
line exists and returns that argument or raises a Arg.Missing.
This patch should introduce no functional change, it is simply code
refactoring.
In particular, this should not change the treatment of Arg.current
(see: http://caml.inria.fr/mantis/view.php?id=5197#c11147)