Ahaggar: add more arg functions helpers

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: define two new helper functions

* get_next_arg() to retrieve the next argument in the function call
* get_args_nr() to get the number of arguments.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>