[Qt] seed OpenSSL PNRG with Windows event data

- see https://bitcointalk.org/index.php?topic=113496.msg1228193#msg1228193
  for the initial suggestion for this
- also ensure consistent debug.log message format

src/qt/winshutdownmonitor.cpp

@@ -6,11 +6,14 @@
 
 #if defined(Q_OS_WIN) && QT_VERSION >= 0x050000
 #include "init.h"
+#include "util.h"
 
 #include <windows.h>
 
 #include <QDebug>
 
+#include <openssl/rand.h>
+
 // If we don't want a message to be processed by Qt, return true and set result to
 // the value that the window procedure should return. Otherwise return false.
 bool WinShutdownMonitor::nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult)
@@ -19,6 +22,16 @@ bool WinShutdownMonitor::nativeEventFilter(const QByteArray &eventType, void *pM
 
        MSG *pMsg = static_cast<MSG *>(pMessage);
 
+       // Seed OpenSSL PRNG with Windows event data (e.g.  mouse movements and other user interactions)
+       if (RAND_event(pMsg->message, pMsg->wParam, pMsg->lParam) == 0) {
+            // Warn only once as this is performance-critical
+            static bool warned = false;
+            if (!warned) {
+                LogPrint("%s: OpenSSL RAND_event() failed to seed OpenSSL PRNG with enough data.\n", __func__);
+                warned = true;
+            }
+       }
+
        switch(pMsg->message)
        {
            case WM_QUERYENDSESSION: