A-Netz / B-Netz: Add function to change state, debug with channel info

Change state uses a function rather than directly setting state
variable.

Some debug commands now use channel number. (for multi-trx setups)

src/anetz/anetz.h

@@ -8,6 +8,7 @@ enum dsp_mode {
 };
 
 enum anetz_state {
+	ANETZ_NULL = 0,
 	ANETZ_FREI,		/* sending 2280 Hz tone */
 	ANETZ_GESPRAECH,	/* during conversation */
 	ANETZ_ANRUF,		/* phone is paged */