net: rename Sock::Reset() to Sock::Close() and make it private

Outside of `Sock`, `Sock::Reset()` was used in just one place (in
`i2p.cpp`) which can use the assignment operator instead.

This simplifies the public `Sock` API by having one method less.

src/i2p.cpp

@@ -410,7 +410,7 @@ void Session::Disconnect()
             Log("Destroying session %s", m_session_id);
         }
     }
-    m_control_sock->Reset();
+    m_control_sock = std::make_unique<Sock>(INVALID_SOCKET);
     m_session_id.clear();
 }
 } // namespace sam