output/jack: use jack_client_open() instead of jack_client_new()

jack_client_new() is deprecated.  This requires libjack 0.100
(released nearly 5 years ago).  We havn't been testing older libjack
versions anyway.

As a side effect, there is the new option "autostart".

doc/user.xml

@@ -745,6 +745,17 @@ cd mpd-version</programlisting>
                   Player Daemon".
                 </entry>
               </row>
+              <row>
+                <entry>
+                  <varname>autostart</varname>
+                  <parameter>yes|no</parameter>
+                </entry>
+                <entry>
+                  If set to <parameter>yes</parameter>, then
+                  <filename>libjack</filename> will automatically
+                  launch the JACK daemon.  Disabled by default.
+                </entry>
+              </row>
               <row>
                 <entry>
                   <varname>ports</varname>