Kernel: Add a LoopbackAdapter for talking to yourself via 127.0.0.1.

Choosing adapter for transmit is done by adapter_for_route_to(IPv4Address).
This is just hard-coded logic right now but can be expanded to support a
proper routing table.

Also start moving kernel networking code into Kernel/Net/.

Kernel/Net/.gitignore

@@ -0,0 +1,2 @@
+*.o
+*.d

Kernel/Net/LoopbackAdapter.cpp

@@ -0,0 +1,24 @@
+#include <Kernel/Net/LoopbackAdapter.h>
+
+LoopbackAdapter& LoopbackAdapter::the()
+{
+    static LoopbackAdapter* the;
+    if (!the)
+        the = new LoopbackAdapter;
+    return *the;
+}
+
+LoopbackAdapter::LoopbackAdapter()
+{
+    set_ipv4_address({ 127, 0, 0, 1 });
+}
+
+LoopbackAdapter::~LoopbackAdapter()
+{
+}
+
+void LoopbackAdapter::send_raw(const byte* data, int size)
+{
+    dbgprintf("LoopbackAdapter: Sending %d byte(s) to myself.\n", size);
+    did_receive(data, size);
+}

Kernel/Net/LoopbackAdapter.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <Kernel/NetworkAdapter.h>
+
+class LoopbackAdapter final : public NetworkAdapter {
+public:
+    static LoopbackAdapter& the();
+
+    virtual ~LoopbackAdapter() override;
+
+    virtual void send_raw(const byte*, int) override;
+    virtual const char* class_name() const override { return "LoopbackAdapter"; }
+
+private:
+    LoopbackAdapter();
+};

Kernel/Net/Routing.cpp

@@ -0,0 +1,10 @@
+#include <Kernel/Net/Routing.h>
+#include <Kernel/Net/LoopbackAdapter.h>
+
+NetworkAdapter* adapter_for_route_to(const IPv4Address& ipv4_address)
+{
+    // FIXME: Have an actual routing table.
+    if (ipv4_address == IPv4Address(127, 0, 0, 1))
+        return &LoopbackAdapter::the();
+    return NetworkAdapter::from_ipv4_address(IPv4Address(192, 168, 5, 2));
+}

Kernel/Net/Routing.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include <Kernel/NetworkAdapter.h>
+
+NetworkAdapter* adapter_for_route_to(const IPv4Address&);