Allow multi-sign to be enabled at runtime:

This lets unit tests exercise multi-sign interfaces
without having to set RIPPLE_MULTI_SIGN_ENABLE.

src/ripple/app/tx/impl/SetSignerList.cpp

@@ -357,6 +357,8 @@ transact_SetSignerList (
     TransactionEngineParams params,
     TransactionEngine* engine)
 {
+    if (! engine->enableMultiSign())
+        return temDISABLED;
     return SetSignerList (txn, params, engine).apply ();
 }