audio: update usage messages

audio.c

@@ -64,4 +64,10 @@ void audio_ls_outputs(void) {
     printf("Available audio outputs:\n");
     for (out=outputs; *out; out++)
         printf("    %s%s\n", (*out)->name, out==outputs ? " (default)" : "");
+    
+    for (out=outputs; *out; out++) {
+        printf("\n");
+        printf("Options for output %s:\n", (*out)->name);
+        (*out)->help();
+    }
 }

audio_dummy.c

@@ -69,7 +69,7 @@ static void stop(void) {
 }
 
 static void help(void) {
-    printf("There are no options for dummy audio.\n");
+    printf("    There are no options for dummy audio.\n");
 }
 
 audio_output audio_dummy = {

audio_pipe.c

@@ -72,7 +72,7 @@ static void deinit(void) {
 }
 
 static void help(void) {
-    printf("pipe takes 1 argument: the name of the FIFO to write to.\n");
+    printf("    pipe takes 1 argument: the name of the FIFO to write to.\n");
 }
 
 audio_output audio_pipe = {