add missing Boost Thread join_all() call during shutdown

- fixes #3037 by adding missing join_all() call and brings bitcoind
  shutdown code in line with Bitcoin-Qt shutdown code
- added a comment for the if (!fRet) case

src/bitcoind.cpp

@@ -9,15 +9,18 @@
 
 void DetectShutdownThread(boost::thread_group* threadGroup)
 {
-    bool shutdown = ShutdownRequested();
+    bool fShutdown = ShutdownRequested();
     // Tell the main threads to shutdown.
-    while (!shutdown)
+    while (!fShutdown)
     {
         MilliSleep(200);
-        shutdown = ShutdownRequested();
+        fShutdown = ShutdownRequested();
     }
     if (threadGroup)
+    {
         threadGroup->interrupt_all();
+        threadGroup->join_all();
+    }
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -107,10 +110,16 @@ bool AppInit(int argc, char* argv[])
     } catch (...) {
         PrintExceptionContinue(NULL, "AppInit()");
     }
-    if (!fRet) {
+
+    if (!fRet)
+    {
         if (detectShutdownThread)
             detectShutdownThread->interrupt();
+
         threadGroup.interrupt_all();
+        // threadGroup.join_all(); was left out intentionally here, because we didn't re-test all of
+        // the startup-failure cases to make sure they don't result in a hang due to some
+        // thread-blocking-waiting-for-another-thread-during-startup case
     }
 
     if (detectShutdownThread)