This commit makes it possible to specify the Kilo interface name. If the
specified interface exists, it will be used; if it does not exist, Kilo
will create it. If the interface already existed, then it will not be
deleted on shutdown; otherwise Kilo will destroy the interface.
Fixes: https://github.com/squat/kilo/issues/8
Addresses: 1/2 of https://github.com/squat/kilo/issues/17
We need to defensively deduplicate peer allowed IPs.
If two peers claim the same IP, the WireGuard configuration
could flap, causing the interface to churn.
