command: allow clients to subscribe to specific idle events

The client may provide the names of idle events as arguments to the
"idle" command to inform MPD that it is only interested in these
events.

src/idle.c

@@ -30,6 +30,18 @@
 static unsigned idle_flags;
 static pthread_mutex_t idle_mutex = PTHREAD_MUTEX_INITIALIZER;
 
+static const char *const idle_names[] = {
+	"database",
+	"stored_playlist",
+	"playlist",
+	"player",
+	"mixer",
+	"output",
+	"options",
+	"elapsed",
+        NULL
+};
+
 void
 idle_add(unsigned flags)
 {
@@ -54,3 +66,9 @@ idle_get(void)
 
 	return flags;
 }
+
+const char*const*
+idle_get_names(void)
+{
+        return idle_names;
+}