Fix -logtimestamps to only print time prefix once per output line

Incorporate BlueMatt's fix to only timestamp upon new line, and
move -logtimestamp checking outside OutputDebugPrintF() to better
future-proof it.

util.h

@@ -148,6 +148,7 @@ extern bool fCommandLine;
 extern string strMiscWarning;
 extern bool fTestNet;
 extern bool fNoListen;
+extern bool fLogTimestamps;
 
 void RandAddSeed();
 void RandAddSeedPerfmon();