[client] audio: minimize LGMP playback latency

Scope the legacy latency offset to classic SPICE and use the LGMP
poll bound as its fixed arrival reserve. Keep learned jitter additive.

Select provider clock feedback only when the current remote endpoint
advertises it, avoiding a failed startup attempt on broker v1.

client/src/config.c

@@ -517,7 +517,7 @@ static struct Option options[] =
   {
     .module         = "audio",
     .name           = "latencyOffset",
-    .description    = "Latency offset added to the calculated minimum in milliseconds",
+    .description    = "Extra classic SPICE playback latency in milliseconds",
     .type           = OPTION_TYPE_INT,
     .value.x_int    = 6
   },