ipv6-update: support dhcpv6-client-lease script...

... by adding a provide, and handling global variable
$DHCPv6ClientLeaseVars.

dhcpv6-client-lease.rsc

@@ -33,8 +33,7 @@
     "na-address"=$"na-address";
     "na-valid"=$"na-valid";
     "pd-prefix"=$"pd-prefix";
-    "pd-valid"=$"pd-valid";
-    "options"=$"options" };
+    "pd-valid"=$"pd-valid" };
 
   :local RunOrder ({});
   :foreach Script in=[ /system/script/find where source~("\n# provides: dhcpv6-client-lease\\b") ] do={

doc/dhcpv6-client-lease.md

@@ -34,7 +34,6 @@ available with a global variable. This code is required in child script:
     :local NaValid [ $EitherOr $"na-valid" ($DHCPv6ClientLeaseVars->"na-valid") ];
     :local PdPrefix [ $EitherOr $"pd-prefix" ($DHCPv6ClientLeaseVars->"pd-prefix") ];
     :local PdValid [ $EitherOr $"pd-valid" ($DHCPv6ClientLeaseVars->"pd-valid") ];
-    :local Options [ $EitherOr $"options" ($DHCPv6ClientLeaseVars->"options") ];
 
 The values are available under different name then, use `$PdPrefix` instead
 of `$"pd-prefix"`, and so on. The resulting script supports both, being a