Start NQPTP only on a specific interface

[marcnnn]

Based on this, mikebrady/shairport-sync#1956

I will use this Issue to discuss how to implement it.

So it will be possible to merge it.

[mikebrady]

That’s fine, thanks. I was briefly thinking about this, and it would be nice to be able to add more than one interface. Maybe with an -i command line option which can be repeated. I have some ideas for the code as well but I’m not near a computer just now…