Do the Right Thing for the host_(get/set)_pstate

Do the Right Thing for the host_(get/set)_pstate

- The prefered public interface is in simgrid::Host
  - the getter directly calls pimpl_cpu->getPState()
  - the setter simcalls to the pimpl, without diverting to simix
  - (this will be automatically forwarded to Java with SWIG one day)
- The C public interface is sg_host_(get/set)_pstate
  - MSG_host_(get/set)_pstate is #defined to this for compatibility
  - SMPI adapts the interface in smpi_host_(get/set)_pstate to make
      the receiver host implicit.
    I don't see the point but don't want to mess with SMPI yet.

One day, the whole of SimGrid will be organized this way. One day.