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/TxFormats.h

@@ -58,6 +58,7 @@ enum TxType {
 
     ttAMENDMENT = 100,
     ttFEE = 101,
+    ttUNL_MODIFY = 102,
 };
 
 /** Manages the list of known transaction formats.