Testing infrastructure: mocktime fixes

New, undocumented-on-purpose -mocktime=timestamp command-line
argument to startup with mocktime set. Needed because
time-related blockchain sanity checks are done on startup, before a
test has a chance to make a setmocktime RPC call.

And changed the setmocktime RPC call so calling it will not result in
currently connected peers being disconnected due to inactivity timeouts.

src/init.cpp

@@ -981,6 +981,9 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     fAlerts = GetBoolArg("-alerts", DEFAULT_ALERTS);
 
+    // Option to startup with mocktime set (used for regression testing):
+    SetMockTime(GetArg("-mocktime", 0)); // SetMockTime(0) is a no-op
+
 #ifdef ENABLE_MINING
     if (mapArgs.count("-mineraddress")) {
         CBitcoinAddress addr;