Kernel: Give each FileDescriptor a chance to co-open sockets.

Track how many fds are open for a socket's Accepted and Connected roles.
This allows fork() to clone a socket fd without a subsequent close() walking
all over the parent process's fd.

Kernel/Socket.cpp

@@ -36,7 +36,6 @@ bool Socket::listen(int backlog, int& error)
         return false;
     }
     m_backlog = backlog;
-    m_listening = true;
     kprintf("Socket{%p} listening with backlog=%d\n", this, m_backlog);
     return true;
 }