Kernel: Sending a signal to yourself should block until dispatch.

By moving the sending (and receiving) thread into the BlockedSignal state,
we ensure that the thread doesn't continue executing until the signal
has been dispatched.

Kernel/Process.cpp

@@ -1232,7 +1232,7 @@ int Process::sys$kill(pid_t pid, int signal)
     }
     if (pid == m_pid) {
         current->send_signal(signal, this);
-        Scheduler::yield();
+        current->block(Thread::State::BlockedSignal);
         return 0;
     }
     InterruptDisabler disabler;