Changes for SLIP & TUN connectivity

- Define TUN or TAP settings/config based on LLH define
- Add SLIP_Gateway example, demonstrating how to configure Arduino as a
gateway for a SLIP device. Should be compatible with any SLIP enabled
device that an Arduino can connect to via USB
- Add SLIP_InteractiveServer to demonstrate how to use RF24Mesh to
interact with a TUN or SLIP gateway like the SLIP_Gateway example
Fix instructions for SLIP_Gateway example
Update SLIP_Gateway example to make it clearer

- Move all RF24Network functions to main loop, from slipdev file
Minor changes