output: added option "always_on" for radio stations

Did you ever accidently click "stop" while feeding a radio station?
This option sets the output device to "pause" to disable the "close"
method.  It falls back to "pause" then, which is specific to the
plugin.  Some plugins implement it by feeding silence.

doc/user.xml

@@ -337,6 +337,18 @@ cd mpd-version</programlisting>
                 enabled.
               </entry>
             </row>
+            <row>
+              <entry>
+                <varname>always_on</varname>
+                  <parameter>yes|no</parameter>
+              </entry>
+              <entry>
+                If set to "yes", then MPD attempts to keep this audio
+                output always open.  This may be useful for streaming
+                servers, when you don't want to disconnect all
+                listeners even when playback is accidently stopped.
+              </entry>
+            </row>
             <row>
               <entry>
                 <varname>mixer_type</varname>