openwrt: save cache to disk when reboot system

package/openwrt/control/conffiles

@@ -2,3 +2,5 @@
 /etc/smartdns/address.conf
 /etc/smartdns/blacklist-ip.conf
 /etc/smartdns/custom.conf
+/etc/smartdns/domain-block.list
+/etc/smartdns/domain-forwarding.list

package/openwrt/control/prerm

@@ -19,4 +19,6 @@
 default_prerm $0 $@
 rm /var/etc/smartdns.conf -f
 rm /var/etc/smartdns/smartdns.conf -f
+rm /var/log/smartdns/ -fr
+rm /etc/smartdns/smartdns.cache -f
 exit 0