Fix app-lifetime thread regression on JVM backend.

The thread pool scheduler wants its threads to be background ones, but
they were not marked as such. This caused hangs at process end.

src/vm/jvm/runtime/org/perl6/nqp/runtime/Ops.java

@@ -4197,6 +4197,7 @@ public void run() {
     public static SixModelObject newthread(SixModelObject code, long appLifetime, ThreadContext tc) {
         SixModelObject thread = tc.gc.Thread.st.REPR.allocate(tc, tc.gc.Thread.st);
         ((VMThreadInstance)thread).thread = new Thread(new CodeRunnable(tc.gc, thread, code));
+        ((VMThreadInstance)thread).thread.setDaemon(appLifetime != 0);
         return thread;
     }