Only create the avahi webamin config if it doesn't exist

src/freedombone-utils-webadmin

@@ -27,6 +27,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 function web_admin_avahi {
+    if [ -f /etc/avahi/services/webadmin.service ]; then
+        return
+    fi
+
     { echo '<?xml version="1.0" standalone="no"?><!--*-nxml-*-->';
       echo '<!DOCTYPE service-group SYSTEM "avahi-service.dtd">';
       echo '<service-group>';