make msg_process_t point onto s4u::Actor*

make msg_process_t point onto s4u::Actor*

One day, all msg_blah_t will point to s4u::Blah*.

(I'm glad it worked: that's the second time I attempt this improvement!)

Other changes on the way:
- change the prototype of MSG_host_get_process_list():
  it was returning the internal list, but this one contains smx_actor_t,
  not msg_process_t as we can handle from the public interface
- convert a C test in teshsuite to C++ (even if I could have avoided this)