The route creation for the `tun` device was augmented in #1469 but
didn't account for adding IPv4 vs. IPv6 routes. There are 2 primary
changes as a result:
* Ensure that either `-inet` or `-inet6` was used in the
[`route(8)`](https://man.openbsd.org/route) command
* Use either the `localAddr4` or `localAddr6` for the gateway argument
depending which destination network is being added
The basis for the approach is based on the implementation from
`router_userspace_bsd.go`, including the `inet()` helper function.
Fixes#2048
References #1469
Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>
a49df5cfda