* include/vlc_block.h, modules/codec/libmpeg2.c: re-added the discontinuity flag in block_t and re-enabled the discontinuity gestion in the libmpeg2 codec.
* include/vlc_block_helper.h: implemented bytestream reading helper functions for chained blocks.
* modules/codec/a52.c: modified to use the bytestream reading functions.
  The flexibility added by these functions makes the code simpler and better at detecting synchro code emulations.