vss: unify calls to the FEC layer

vss: unify calls to the FEC layer

This reorganizes the internal vss data structures a bit to unify calls to the
FEC layer across different senders:
 * each sender supplies its sender_client data via the 'sc' pointer;
 * the init_fec()/send_fec() function pointers are now parameters
   (so that a global variable is not needed);
 * both these functions then operate on the supplied sender_client data;
 * the transport layer can then interpret the sender_client data in
   specific ways, e.g. by accessing sc->private_data;
 * a new 'ready' flag has been added to track transport-layer state;
 * max_slice_size as paramter has been restored (for overriding MPS value).

Note: this is one patch out of several. It comments out now-incompatible
      changes in the API for the sake of being able to compile. However,
      the new global API requires the whole set of patches to be applied.