SERVER-2833 -- shut down cleanly on OS or Service Controller shutdown event

Added code to log shutdown events from the Windows Service Controller
and to respond correctly to those events.  Shutdown from the console
on logoff or shutdown callbacks (stop ignoring them).

db/instance.cpp

@@ -927,6 +927,15 @@ namespace mongo {
         catch (...) { }
 #endif
 
+#ifdef _WIN32
+        // Windows Service Controller wants to be told when we are down,
+        //  so don't call ::exit() yet, or say "really exiting now"
+        //
+        if ( rc == EXIT_WINDOWS_SERVICE_STOP ) {
+            if ( c ) c->shutdown();
+            return;
+        }
+#endif
         tryToOutputFatal( "dbexit: really exiting now" );
         if ( c ) c->shutdown();
         ::exit(rc);