decoder/mikmod: Add loop configuration parameter

This patch allows the user to configure the mikmod decoder plugin to loop
modules. It adds a configuration parameter to the mikmod decoder called
"loop" which can be "no" (the old behaviour, default) or "yes" to allow
modules to use backward loops.

doc/user.xml

@@ -976,6 +976,16 @@ systemctl start mpd.socket</programlisting>
               </row>
             </thead>
             <tbody>
+              <row>
+                <entry>
+                  <varname>loop</varname>
+                  <parameter>yes|no</parameter>
+                </entry>
+                <entry>
+                  Allow backward loops in modules.  Default is
+		  <parameter>no</parameter>.
+                </entry>
+              </row>
               <row>
                 <entry>
                   <varname>sample_rate</varname>