Build for openbsd

[nbrownus]

Adds a build for openbsd and restructures a few things to make supporting bsd family of OS easier.

We should think about this change in relation to #808

openbsd was requested in #806

[johnmaguire]

Rebased on master in order to pull in changes from #808

Would be nice to get this merged. We have a user on Nebula OSS Slack who'd like to try running on OpenBSD: https://nebulaoss.slack.com/archives/CS01XE0KZ/p1683756250949619?thread_ts=1683753436.867029&cid=CS01XE0KZ

[johnmaguire]

@wadey did a little bit of work towards this goal in https://github.com/wadey/nebula/tree/smoke-openbsd

To run it with Virtualbox and Vagrant, use make smoke-vagrant/openbsd-amd64

It's currently failing with:

time="2023-01-23T21:48:50Z" level=fatal msg="failed to run 'route add': exit status 1"

@wadey thinks the following may solve the issue: sudo /sbin/route -n add -inet 192.168.100.3/24 192.168.100.3

[vagrant@openbsd7 ~]$ route get 192.168.100.1
   route to: 192.168.100.1
destination: 192.168.100.0
       mask: 255.255.255.0
    gateway: 192.168.100.3
  interface: tun0
 if address: 192.168.100.3
   priority: 8 (static)
      flags: <UP,GATEWAY,DONE,STATIC>
     use       mtu    expire
       1         0         0

[6167656e74323431]

I think the command you probably want here is /sbin/route -n add -net 192.168.100.3/24 -link -iface tun0.

[wadey]

We should add release-openbsd to the Makefile and add it to the release workflow as well here:

• nebula/.github/workflows/release.yml

  Line 22 in e5af94e

  make BUILD_NUMBER="${GITHUB_REF#refs/tags/v}" release-linux release-freebsd