[SMPI] Allow privatization in a more generic way

[SMPI] Allow privatization in a more generic way

1. We now use a std::set instead of a static array
2. simgrid::smpi::Process::Init() now initializes the global data segment
   by using a (clean) copy of the data segment that we backuped at startup.
   (This is done by calling smpi_init_global_memory_segment_process(), a function
   that contains mostly code that was looped-over before.)