Remove a level of constness for argv parameter of sg_actor_start/create.

Remove a level of constness for argv parameter of sg_actor_start/create.

Aligns with the signature of main(), and allows to write without cast:

    int main(int argc, char *argv[]) {
        sg_actor_start(NULL, NULL, argc, argv);
        ...

The main drawback is that we have to add an explicit cast in our constrained
examples.