Make the "default" string a valid device name that has the same effect

as NULL has. This will (hopefully) simplify ports where the user
passes the device string.

src/include/sndio.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: sndio.h,v 1.5 2012/04/11 06:05:43 ratchov Exp $	*/
+/*	$OpenBSD: sndio.h,v 1.6 2012/05/23 19:25:11 ratchov Exp $	*/
 /*
  * Copyright (c) 2008 Alexandre Ratchov <alex@caoua.org>
  *
@@ -19,6 +19,12 @@
 
 #include <sys/param.h>
 
+/*
+ * default audio device and MIDI port
+ */
+#define SIO_DEVANY	"default"
+#define MIO_PORTANY	"default"
+
 /*
  * private ``handle'' structure
  */