wgenine/router: fix OpenBSD route creation

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>
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