ao_rsound: allow setting the host

Completely untested (rsound dev libs unavailable on my system). Trivial
enough that it's very likely that it'll just work. No port selection,
but could be added by parsing it as part of the device name.

Should fix #4714.

DOCS/man/ao.rst

@@ -216,7 +216,8 @@ Available audio output drivers are:
         it will write a WAVE header every time the file is opened.
 
 ``rsound``
-    Audio output to an RSound daemon
+    Audio output to an RSound daemon. Use ``--audio-device=rsound/<hostname>``
+    to set the host name (with ``<hostname>`` replaced, without the ``< >``).
 
     .. note:: Completely useless, unless you intend to run RSound. Not to be
               confused with RoarAudio, which is something completely

audio/out/ao_rsound.c

@@ -64,6 +64,9 @@ static int init(struct ao *ao)
     if (rsd_init(&priv->rd) < 0)
         return -1;
 
+    if (ao->device)
+        rsd_set_param(priv->rd, RSD_HOST, ao->device);
+
     // Actual channel layout unknown.
     struct mp_chmap_sel sel = {0};
     mp_chmap_sel_add_waveext_def(&sel);