MEDIUM: arg: make make_arg_list() stop after its own arguments

MEDIUM: arg: make make_arg_list() stop after its own arguments

The main problem we're having with argument parsing is that at the
moment the caller looks for the first character looking like an end
of arguments (')') and calls make_arg_list() on the sub-string inside
the parenthesis.

Let's first change the way it works so that make_arg_list() also
consumes the parenthesis and returns the pointer to the first char not
consumed. This will later permit to refine each argument parsing.

For now there is no functional change.