This commit updates the Interface.Inside type to be a new interface
type instead of a *Tun. This will allow for an inside interface
that does not use a tun device, such as a single-binary client that
can run without elevated privileges.
Add support for freebsd. You have to set `tun.dev` in your config. The second pass of this would be to remove the exec calls and use ioctl(2) and route(4) instead, but we can do that in a second PR.
Co-authored-by: Wade Simmons <wade@wades.im>