core/sockets: Add missing socket services and functions from switchbrew

- Add Unknown39 and Unknown40 functions to BSD service for [20.0.0+]
- Implement bsd:nu service with ISfUserService and ISfAssignedNetworkInterfaceService
- Add dns:priv and ethc:c/ethc:i service stubs
- Update CMakeLists.txt to include new socket service files
- All new functions include basic stub implementations following existing patterns

This completes the socket services implementation based on switchbrew.org documentation.

Signed-off-by: Zephyron <zephyron@citron-emu.org>

src/core/hle/service/sockets/ethc.h

@@ -0,0 +1,44 @@
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "core/hle/service/service.h"
+
+namespace Core {
+class System;
+}
+
+namespace Service::Sockets {
+
+class ETHC_C final : public ServiceFramework<ETHC_C> {
+public:
+    explicit ETHC_C(Core::System& system_);
+    ~ETHC_C() override;
+
+private:
+    // Ethernet controller service methods (ethc:c)
+    // Based on switchbrew documentation - functions are undocumented so basic stubs are provided
+    void Unknown0(HLERequestContext& ctx);
+    void Unknown1(HLERequestContext& ctx);
+    void Unknown2(HLERequestContext& ctx);
+    void Unknown3(HLERequestContext& ctx);
+    void Unknown4(HLERequestContext& ctx);
+};
+
+class ETHC_I final : public ServiceFramework<ETHC_I> {
+public:
+    explicit ETHC_I(Core::System& system_);
+    ~ETHC_I() override;
+
+private:
+    // Ethernet controller interface service methods (ethc:i)
+    // Based on switchbrew documentation - functions are undocumented so basic stubs are provided
+    void Unknown0(HLERequestContext& ctx);
+    void Unknown1(HLERequestContext& ctx);
+    void Unknown2(HLERequestContext& ctx);
+    void Unknown3(HLERequestContext& ctx);
+    void Unknown4(HLERequestContext& ctx);
+};
+
+} // namespace Service::Sockets