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/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));
+}