linux: opt out of naming your tun device yourself

[JackDoan]

This allows you to end your tun.dev name setting with %d to tell Nebula "please find the lowest-numbered interface that matches this pattern and use it"

I'm honestly not sure what would be needed to bring this to other platforms. I think Mac OS mostly doesn't care. Windows seems like it will need different handling as well, but won't break with this change.