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/FileDescriptor.h

@@ -86,7 +86,7 @@ public:
 
     void set_original_inode(Badge<VFS>, RetainPtr<Inode>&& inode) { m_inode = move(inode); }
 
-    void set_socket_role(SocketRole role) { m_socket_role = role; }
+    void set_socket_role(SocketRole);
 
 private:
     friend class VFS;