(I've chosen 32 now), that doesn't use any memcpy anymore and is also much
faster. It is ready now for cell positioning in multi-angle DVD (to come
soon).

-DVD specific netlist adapted to 2048 bytes long blocks that contain
several data_packets.

-Modification in mpeg_system.c to use p_payload_start instead of
p_buffer (necessary for DVD plugin). It does not break other plugins
since they set p_payload_start to p_buffer

-New field i_read_once in input_thread_s to be able to read more than
the TS related 7 packets ; i_read_once is set to INPUT_READ_ONCE by
defaults but plugins can change it during initialisation.

Please check that nothing is broken :)