security: support dropping root, and no root privileges required for ping

etc/smartdns/smartdns.conf

@@ -4,6 +4,12 @@
 #   server-name smartdns
 #
 
+# dns server run ser
+# user [username]
+# example: run as nobody
+#   user nobody
+#
+
 # Include another configuration options
 # conf-file [file]
 # conf-file blacklist-ip.conf
@@ -90,6 +96,7 @@ cache-size 16384
 
 # Enable IPV4, IPV6 dual stack IP optimization selection strategy
 # dualstack-ip-selection-threshold [num] (0~1000)
+# dualstack-ip-allow-force-AAAA [yes|no]
 # dualstack-ip-selection [yes|no]
 # dualstack-ip-selection yes