Add option -alerts to opt out of alert system

Make it possible to opt-out of the centralized alert system by providing
an option `-noalerts` or `-alerts=0`. The default remains unchanged.

This is a gentler form of #6260, in which I went a bit overboard by
removing the alert system completely.

I intend to add this to the GUI options in another pull after this.

Github-Pull: #6274
Rebased-From: 02a6702a82

src/main.cpp

@@ -61,6 +61,7 @@ bool fCheckBlockIndex = false;
 bool fCheckpointsEnabled = true;
 size_t nCoinCacheUsage = 5000 * 300;
 uint64_t nPruneTarget = 0;
+bool fAlerts = DEFAULT_ALERTS;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
 CFeeRate minRelayTxFee = CFeeRate(1000);
@@ -4605,7 +4606,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
     }
 
 
-    else if (strCommand == "alert")
+    else if (fAlerts && strCommand == "alert")
     {
         CAlert alert;
         vRecv >> alert;