Kernel: Bring up enough networking code that we can respond to ARP requests.

This is all pretty rickety but we can now respond to "arping" from the host
while running inside QEMU. Very cool. :^)

Kernel/NetworkAdapter.h

@@ -4,6 +4,7 @@
 #include <AK/SinglyLinkedList.h>
 #include <AK/Types.h>
 #include <Kernel/MACAddress.h>
+#include <Kernel/IPv4Address.h>
 #include <Kernel/ARPPacket.h>
 
 class NetworkAdapter {
@@ -12,6 +13,9 @@ public:
 
     virtual const char* class_name() const = 0;
     MACAddress mac_address() { return m_mac_address; }
+    IPv4Address ipv4_address() const { return m_ipv4_address; }
+
+    void set_ipv4_address(const IPv4Address&);
 
     void send(const MACAddress&, const ARPPacket&);
 
@@ -25,5 +29,6 @@ protected:
 
 private:
     MACAddress m_mac_address;
+    IPv4Address m_ipv4_address;
     SinglyLinkedList<ByteBuffer> m_packet_queue;
 };