dbus: simplify config file

distro/common/cz.nic.knotd.conf

 <!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
 <busconfig>
-  <policy context="default">
-    <deny own="cz.nic.knotd" />
-    <deny send_destination="cz.nic.knotd" />
-    <allow receive_sender="cz.nic.knotd" />
-  </policy>
-  <policy group="knot">
+  <policy user="knot">
     <allow own="cz.nic.knotd" />
-    <allow send_destination="cz.nic.knotd" />
+  </policy>
+  <policy context="default">
     <allow receive_sender="cz.nic.knotd" />
   </policy>
 </busconfig>