wg: Use ENETUNREACH when transmitting to a non-existent peer

The old errno value used is specifically for Capsicum and shouldn't be
co-opted in this way. It has special handling in the generic syscall
layer (see syscallret()). OpenBSD returns ENETUNREACH in this case;
let's do the same thing.

Reviewed by: kevans, imp
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D44582