Small code cleaning in the iface package. These changes necessary to
get a clean code in case if we involve more platforms. The OS related
functions has been distributed into separate files and it has been
mixed with not OS related logic. The goal is to get a clear picture
of the layer between WireGuard and business logic.
* Disable upstream DNS resolver after several tries and fails
* Add tests for upstream fails
* Use an extra flag to disable domains in DNS upstreams
* Fix hashing IPs of nameservers for updates.
Added host configurators for Linux, Windows, and macOS.
The host configurator will update the peer system configuration
directing DNS queries according to its capabilities.
Some Linux distributions don't support split (match) DNS or custom ports,
and that will be reported to our management system in another PR
