Kernel: Add Thread::block_until(Condition).

Replace the class-based snooze alarm mechanism with a per-thread callback.
This makes it easy to block the current thread on an arbitrary condition:

    void SomeDevice::wait_for_irq() {
        m_interrupted = false;
        current->block_until([this] { return m_interrupted; });
    }
    void SomeDevice::handle_irq() {
        m_interrupted = true;
    }

Use this in the SB16 driver, and in NetworkTask :^)

Kernel/Thread.cpp

@@ -108,10 +108,10 @@ void Thread::unblock()
     set_state(Thread::Runnable);
 }
 
-void Thread::snooze_until(Alarm& alarm)
+void Thread::block_until(Function<bool()>&& condition)
 {
-    m_snoozing_alarm = &alarm;
-    block(Thread::BlockedSnoozing);
+    m_block_until_condition = move(condition);
+    block(Thread::BlockedCondition);
     Scheduler::yield();
 }
 
@@ -179,8 +179,8 @@ const char* to_string(Thread::State state)
         return "Connect";
     case Thread::BlockedReceive:
         return "Receive";
-    case Thread::BlockedSnoozing:
-        return "Snoozing";
+    case Thread::BlockedCondition:
+        return "Condition";
     case Thread::__Begin_Blocked_States__:
     case Thread::__End_Blocked_States__:
         break;