[cppmyth] Query IP and port of slave backend

- Myth::Control::GetBackendServerIP6()
- Myth::Control::GetBackendServerIP()
- Myth::Control::GetBackendServerPort()
[mythtv] Slave Backend addr: Using Myth::Control

- Myth::Control::GetBackendServerIP6()
- Myth::Control::GetBackendServerIP()
- Myth::Control::GetBackendServerPort()