prevent `innernet` binary changing interface when run on the `innernet-server` machine

[mcginty]

Reported by @amarao that running innernet on the machine that is the coordinating server causes all kinds of issues with the interface, which we should outright prevent and put a warning message in the binary itself.

[amarao]

innernet fetch, to be precise.

[mcginty]

@amarao does that mean that there was both an /etc/innernet-server/foo.conf and an /etc/innernet/foo.conf? Otherwise, innernet should say that no interface config was found when running innernet fetch.

[amarao]

yes, I made both. I use innernet on innernet-server host to fetch peers and associations.