* plugins/a52_system/Makefile, plugins/a52_system/a52_system.c: a52 input
    module.
  * plugins/ac3_spdif/ac3_spdif.c: added a little mwait to avoid a flood from
    the a52 input module.

Note: the new module aims at reading ac3 streams but it works only with the
ac3_spdif "decoder" since it is the only one which dates each frame contrary
to the others which can't avoid a flood from the input module.