Implement negative UNL functionality:

This change can help improve the liveness of the network during periods of network
instability, by allowing the network to track which validators are presently not online
and to disregard them for the purposes of quorum calculations.

src/ripple/protocol/impl/Feature.cpp

@@ -131,7 +131,9 @@ detail::supportedAmendments()
         "RequireFullyCanonicalSig",
         "fix1781",
         "HardenedValidations",
-        "fixAmendmentMajorityCalc"};
+        "fixAmendmentMajorityCalc",
+        //"NegativeUNL"      // Commented out to prevent automatic enablement
+    };
     return supported;
 }
 
@@ -182,7 +184,8 @@ uint256 const
     featureRequireFullyCanonicalSig = *getRegisteredFeature("RequireFullyCanonicalSig"),
     fix1781                         = *getRegisteredFeature("fix1781"),
     featureHardenedValidations      = *getRegisteredFeature("HardenedValidations"),
-    fixAmendmentMajorityCalc        = *getRegisteredFeature("fixAmendmentMajorityCalc");
+    fixAmendmentMajorityCalc        = *getRegisteredFeature("fixAmendmentMajorityCalc"),
+    featureNegativeUNL              = *getRegisteredFeature("NegativeUNL");
 
 // The following amendments have been active for at least two years. Their
 // pre-amendment code has been removed and the identifiers are deprecated.