Remove ad-hoc handling of possible_in == 0

Just make the code responsible for possible_in > 0 also handle
possible_in == 0 since it nearly does the same thing.

The only difference is that the possible_in == 0 case, by using
FOUNDCFG(), acted as if possible_in was audio_in. The consolidated code
uses FOUNDCFG_IMPRECISE which will add some penalty to the
configurations where desired_in == possible_in != audio_in.

There is thus a small POLICY CHANGE, but the selected configuration will
stay the same unless a better matching configuration is available.

libs/ardour/audio_unit.cc

@@ -1430,23 +1430,6 @@ AUPlugin::can_support_io_configuration (const ChanCount& in, ChanCount& out, Cha
 		}
 
 		/* now allow potentially "imprecise" matches */
-		if (possible_in == 0) {
-			/* no inputs, generators & instruments */
-			if (possible_out == -1 || possible_out == -2) {
-				/* any output configuration possible
-				 * out == -2 is invalid, interpreted as out == -1 */
-				FOUNDCFG (preferred_out);
-				ANYTHINGGOES;
-			} else if (possible_out < -2) {
-				/* variable number of outputs up to -N, */
-				FOUNDCFG (min (-possible_out, preferred_out));
-				UPTO (-possible_out);
-			} else {
-				/* exact number of outputs */
-				FOUNDCFG (possible_out);
-			}
-		}
-
 		if (possible_in == -1 || possible_in == -2) {
 			/* wildcard for input */
 			if (possible_out == possible_in) {
@@ -1469,10 +1452,10 @@ AUPlugin::can_support_io_configuration (const ChanCount& in, ChanCount& out, Cha
 			}
 		}
 
-		if (possible_in < -2 || possible_in > 0) {
+		if (possible_in < -2 || possible_in >= 0) {
 			/* specified number, exact or up to */
 			int32_t desired_in;
-			if (possible_in > 0) {
+			if (possible_in >= 0) {
 				/* configuration can only match possible_in */
 				desired_in = possible_in;
 			} else {