More work on multichannel

Reorder channels to a more common default order.
Only add channel layout when channels > 1
Don't try to link to a session that is still starting
Small sse fixes.

src/modules/module-media-session/audio-dsp.c

@@ -144,6 +144,8 @@ static const struct pw_port_implementation port_implementation = {
 
 static const char *chmap_names[] =
 {
+	"UNK",		/**< unknown */
+	"NA",		/**< unmapped */
 	"MONO",		/**< mono */
 	"FL",		/**< front left */
         "FR",           /**< front right */