MINOR: arg: Be able to forbid unresolved args when building an argument list

MINOR: arg: Be able to forbid unresolved args when building an argument list

In make_arg_list() function, unresolved dependencies are pushed in an
argument list to be resolved later, during the configuration validity
check. It is now possible to forbid such unresolved dependencies by omitting
<al> parameter (setting it to NULL). It is usefull when the parsing context
is not the same than the running context or when the parsing context is lost
after the startup stage. For instance, an argument may be defined in
defaults section during parsing and executed in a frontend/backend section.