MINOR: arg: add a free_args() function to free an args array

MINOR: arg: add a free_args() function to free an args array

make_arg_list() can create an array of arguments, some of which remain
to be resolved, but all users had to deal with their own roll back on
error. Let's add a free_args() function to release all the array's
elements and let the caller deal with the array itself (sometimes it's
allocated in the stack).