LibCore: Add CLocalServer, a server-only local socket class.

Instead of trying to support both client and server in CLocalSocket, let's
have a specialized server class.

The basic usage is:

    CLocalServer server;
    server.listen("/tmp/name-of-portal");
    server.on_ready_to_accept = [&] {
        CLocalSocket* client = server.accept();
        ...
    };

This will make things a lot simpler, since an accepting socket doesn't need
half of the stuff that a regular CIODevice provides. :^)

Libraries/LibCore/CLocalSocket.h

@@ -1,10 +1,15 @@
 #pragma once
+
+#include <AK/Badge.h>
 #include <LibCore/CSocket.h>
 
+class CLocalServer;
+
 class CLocalSocket final : public CSocket {
     C_OBJECT(CLocalSocket)
 public:
     explicit CLocalSocket(CObject* parent = nullptr);
+    CLocalSocket(Badge<CLocalServer>, int fd, CObject* parent = nullptr);
     virtual ~CLocalSocket() override;
 
     virtual bool bind(const CSocketAddress&) override;