debian: fix permissions of /var/lib/qubes/dom0-updates

qubesos/qubes-issues#1029

debian/qubes-core-agent.postinst

@@ -220,6 +220,10 @@ case "${1}" in
 
         chown user:user /home_volatile/user
 
+        if ! dpkg-statoverride --list /var/lib/qubes/dom0-updates >/dev/null 2>&1; then
+            dpkg-statoverride --update --add user user 775 /var/lib/qubes/dom0-updates
+        fi
+
         dpkg-divert --divert /etc/init/serial.conf.qubes-orig --package qubes-core-agent --rename --add /etc/init/serial.conf
 
         # Enable Qubes systemd units

debian/qubes-core-agent.postrm

@@ -48,6 +48,10 @@ if [ "${1}" = "remove" ] ; then
     done
 fi
 
+if [ "$1" = "purge" ]; then
+    dpkg-statoverride --remove /var/lib/qubes/dom0-updates || test $? -eq 2
+fi
+
 # dh_installdeb will replace this with shell code automatically
 # generated by other debhelper scripts.