Change UNL and quorum rules:

* Use fixed size UNL if the total listed validators are below
    threshold.
  * Set quorum to provide Byzantine fault tolerance until a
    threshold of total validators is exceeded, at which time
    quorum is 80%.
  * Ensure that a quorum of 0 cannot be configured.

src/ripple/app/misc/impl/ValidatorList.cpp

@@ -36,7 +36,7 @@ ValidatorList::ValidatorList (
     , publisherManifests_ (publisherManifests)
     , timeKeeper_ (timeKeeper)
     , j_ (j)
-    , quorum_ (minimumQuorum ? *minimumQuorum : 1) // Genesis ledger quorum
+    , quorum_ (minimumQuorum.value_or(1)) // Genesis ledger quorum
     , minimumQuorum_ (minimumQuorum)
 {
 }