Kernel: Snooze the NetworkTask until there are incoming packets to process.

This is accomplished using a new Alarm class and a BlockedSnoozing state.
Basically, you call Process::snooze_until(some_alarm) and then the scheduler
won't wake up the process until some_alarm.is_ringing() returns true.

Kernel/NetworkAdapter.cpp

@@ -25,6 +25,7 @@ NetworkAdapter* NetworkAdapter::from_ipv4_address(const IPv4Address& address)
 }
 
 NetworkAdapter::NetworkAdapter()
+    : m_packet_queue_alarm(*this)
 {
     // FIXME: I wanna lock :(
     ASSERT_INTERRUPTS_DISABLED();
@@ -90,3 +91,8 @@ void NetworkAdapter::set_ipv4_address(const IPv4Address& address)
 {
     m_ipv4_address = address;
 }
+
+bool PacketQueueAlarm::is_ringing() const
+{
+    return m_adapter.has_queued_packets();
+}