packetizer/a52: rewrite header parser
packetizer/a52: rewrite header parser

Rewrite with bit stream helpers using the AC3/EAC3 spec. Add
i_blocks_per_sync_frame, eac3.strmtyp, eac3.i_substreamid that will be used by
the S/PDIF converter.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc