Allow setting minrelaytxfee to 0

Setting minrelaytxfee to 0 will allow all transactions regardless of fee to enter your mempool until it reaches its size limit.  However now that mempool limiting is governed by a separate incrementalrelay fee, it is an unnecessary restriction to prevent a minrelaytxfee of 0.

src/init.cpp

@@ -977,13 +977,14 @@ bool AppInitParameterInteraction()
     // If you are mining, be careful setting this:
     // if you set it to zero then
     // a transaction spammer can cheaply fill blocks using
-    // 1-satoshi-fee transactions. It should be set above the real
+    // 0-fee transactions. It should be set above the real
     // cost to you of processing a transaction.
     if (IsArgSet("-minrelaytxfee"))
     {
         CAmount n = 0;
-        if (!ParseMoney(GetArg("-minrelaytxfee", ""), n) || 0 == n)
+        if (!ParseMoney(GetArg("-minrelaytxfee", ""), n)) {
             return InitError(AmountErrMsg("minrelaytxfee", GetArg("-minrelaytxfee", "")));
+        }
         // High fee check is done afterward in CWallet::ParameterInteraction()
         ::minRelayTxFee = CFeeRate(n);
     } else if (incrementalRelayFee > ::minRelayTxFee) {