split smx_synchro_t into a hierarchy of C++ classes

split smx_synchro_t into a hierarchy of C++ classes

- The change is still rather crude since the code of these objects'
  methods is still spread around in the code (search for dynamic_cast
  in this patch to find the resulting horrors).
  I wanted to keep the patch small to have a chance to finish it
  without revert.

- This breaks MC (at least) because I dunno how to read an object in
  the remote process