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/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);
+}