C-Netz: Make demodulation algorithm (slope or leve) slectable via option

By default (auto), the algorithm is selected, depending on sound card
or SDR usage. For testing and debugging purpose, the algorithm can be
forced.

src/cnetz/fsk_demod.h

@@ -11,6 +11,7 @@ enum fsk_sync {
 };
 
 enum demod_type {
+	FSK_DEMOD_AUTO, /* auto selection of the demod type below */
 	FSK_DEMOD_SLOPE, /* check for highest slope (good for sound cards) */
 	FSK_DEMOD_LEVEL, /* check for zero crossing (good for SDR) */
 };