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/Net/NetworkAdapter.cpp

@@ -33,7 +33,6 @@ NetworkAdapter* NetworkAdapter::from_ipv4_address(const IPv4Address& address)
 }
 
 NetworkAdapter::NetworkAdapter()
-    : m_packet_queue_alarm(*this)
 {
     // FIXME: I wanna lock :(
     all_adapters().resource().set(this);
@@ -106,8 +105,3 @@ void NetworkAdapter::set_interface_name(const StringView& basename)
     builder.append('0');
     m_name = builder.to_string();
 }
-
-bool PacketQueueAlarm::is_ringing() const
-{
-    return m_adapter.has_queued_packets();
-}