*New configuration option to choose audio output format at runtime.

*New configuration option to choose audio output format at runtime.
It allows to select spdif pass-through decoder with --ac3_adec and to
remove the ugly b_ac3 in main.h

As a consequence --spdif doesn't work anymore: you should use --ac3_adec pass

*Fixed a typo in main.c

*Return the option name in error message for unknown option intead of the
argument.