dbus: Allow only root to receive signals

Change the dbus policy file to only allow root applications to receive
signals from wpa_supplicant. This keeps WPS Credentials data secret
from non-root listeners.

wpa_supplicant/dbus-wpa_supplicant.conf

@@ -12,7 +12,7 @@
 
                 <allow send_destination="fi.w1.wpa_supplicant1"/>
                 <allow send_interface="fi.w1.wpa_supplicant1"/>
-
+                <allow receive_sender="fi.w1.wpa_supplicant1" receive_type="signal"/>
         </policy>
         <policy context="default">
                 <deny own="fi.epitest.hostap.WPASupplicant"/>
@@ -22,5 +22,6 @@
                 <deny own="fi.w1.wpa_supplicant1"/>
                 <deny send_destination="fi.w1.wpa_supplicant1"/>
                 <deny send_interface="fi.w1.wpa_supplicant1"/>
+                <deny receive_sender="fi.w1.wpa_supplicant1" receive_type="signal"/>
         </policy>
 </busconfig>