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/CLocalServer.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <LibCore/CNotifier.h>
+#include <LibCore/CObject.h>
+
+class CLocalSocket;
+
+class CLocalServer : public CObject {
+    C_OBJECT(CLocalServer)
+public:
+    explicit CLocalServer(CObject* parent = nullptr);
+    virtual ~CLocalServer() override;
+
+    bool is_listening() const { return m_listening; }
+    bool listen(const String& address);
+
+    CLocalSocket* accept();
+
+    Function<void()> on_ready_to_accept;
+
+private:
+    int m_fd { -1 };
+    bool m_listening { false };
+    OwnPtr<CNotifier> m_notifier;
+};