Kernel: Fix crash when delivering signal to barely created thread

We need to wait until a thread is fully set up and ready for running
before attempting to deliver a signal. Otherwise we may not have a
user stack yet.

Also, remove the Skip0SchedulerPasses and Skip1SchedulerPass thread
states that we don't really need anymore with software context switching.

Fixes the kernel crash reported in #3419

Kernel/Scheduler.cpp

@@ -306,12 +306,6 @@ void Thread::consider_unblock(time_t now_sec, long now_usec)
             unblock();
         return;
     }
-    case Thread::Skip1SchedulerPass:
-        set_state(Thread::Skip0SchedulerPasses);
-        return;
-    case Thread::Skip0SchedulerPasses:
-        set_state(Thread::Runnable);
-        return;
     }
 }