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/IPv4Socket.cpp

@@ -89,6 +89,22 @@ KResult IPv4Socket::bind(const sockaddr* address, socklen_t address_size)
     return protocol_bind();
 }
 
+KResult IPv4Socket::listen(int backlog)
+{
+    int rc = allocate_local_port_if_needed();
+    if (rc < 0)
+        return KResult(-EADDRINUSE);
+
+    if (m_local_address.to_u32() == 0)
+        return KResult(-EADDRINUSE);
+
+    set_backlog(backlog);
+
+    kprintf("IPv4Socket{%p} listening with backlog=%d\n", this, backlog);
+
+    return protocol_listen();
+}
+
 KResult IPv4Socket::connect(FileDescription& description, const sockaddr* address, socklen_t address_size, ShouldBlock should_block)
 {
     if (address_size != sizeof(sockaddr_in))
@@ -157,6 +173,9 @@ ssize_t IPv4Socket::sendto(FileDescription&, const void* data, size_t data_lengt
     if (!adapter)
         return -EHOSTUNREACH;
 
+    if (m_local_address.to_u32() == 0)
+        m_local_address = adapter->ipv4_address();
+
     int rc = allocate_local_port_if_needed();
     if (rc < 0)
         return rc;