Kernel: Refactor TCP/IP stack

This has several significant changes to the networking stack.

* Significant refactoring of the TCP state machine. Right now it's
  probably more fragile than it used to be, but handles quite a lot
  more of the handshake process.
* `TCPSocket` holds a `NetworkAdapter*`, assigned during `connect()` or
  `bind()`, whichever comes first.
* `listen()` is now virtual in `Socket` and intended to be implemented
  in its child classes
* `listen()` no longer works without `bind()` - this is a bit of a
  regression, but listening sockets didn't work at all before, so it's
  not possible to observe the regression.
* A file is exposed at `/proc/net_tcp`, which is a JSON document listing
  the current TCP sockets with a bit of metadata.
* There's an `ETHERNET_VERY_DEBUG` flag for dumping packet's content out
  to `kprintf`. It is, indeed, _very debug_.

Kernel/Net/IPv4SocketTuple.h

@@ -0,0 +1,63 @@
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/SinglyLinkedList.h>
+#include <Kernel/DoubleBuffer.h>
+#include <Kernel/KBuffer.h>
+#include <Kernel/Lock.h>
+#include <Kernel/Net/IPv4.h>
+#include <Kernel/Net/Socket.h>
+
+class IPv4SocketTuple {
+public:
+    IPv4SocketTuple(IPv4Address local_address, u16 local_port, IPv4Address peer_address, u16 peer_port)
+        : m_local_address(local_address)
+        , m_local_port(local_port)
+        , m_peer_address(peer_address)
+        , m_peer_port(peer_port) {};
+
+    IPv4Address local_address() const { return m_local_address; };
+    u16 local_port() const { return m_local_port; };
+    IPv4Address peer_address() const { return m_peer_address; };
+    u16 peer_port() const { return m_peer_port; };
+
+    bool operator==(const IPv4SocketTuple other) const
+    {
+        return other.local_address() == m_local_address && other.local_port() == m_local_port && other.peer_address() == m_peer_address && other.peer_port() == m_peer_port;
+    };
+
+    String to_string() const
+    {
+        return String::format(
+            "%s:%d -> %s:%d",
+            m_local_address.to_string().characters(),
+            m_local_port,
+            m_peer_address.to_string().characters(),
+            m_peer_port);
+    }
+
+private:
+    IPv4Address m_local_address;
+    u16 m_local_port { 0 };
+    IPv4Address m_peer_address;
+    u16 m_peer_port { 0 };
+};
+
+namespace AK {
+
+template<>
+struct Traits<IPv4SocketTuple> : public GenericTraits<IPv4SocketTuple> {
+    static unsigned hash(const IPv4SocketTuple& tuple)
+    {
+        auto h1 = pair_int_hash(tuple.local_address().to_u32(), tuple.local_port());
+        auto h2 = pair_int_hash(tuple.peer_address().to_u32(), tuple.peer_port());
+        return pair_int_hash(h1, h2);
+    }
+
+    static void dump(const IPv4SocketTuple& tuple)
+    {
+        kprintf("%s", tuple.to_string().characters());
+    }
+};
+
+}