defaultroute: fix dual-stack and IPv6-only handling

[zarychtam_plan-b.pwste.edu.pl]

Summary

Since IPv6-only setups are becoming more common, and IPv6 connectivity is often sufficient for tasks such as DNS resolution and NTP time synchronization, we should consider updating this rc.d script.

I have been testing D56637 recently, and if this change is accepted and lands, it will also require an update to the defaultroute script. However, regardless of the status of D56637, the defaultroute script can be made network stack–independent and support both inet and inet6.

If the kernel supports neither INET nor INET6 (which is theoretically possible), the script will return immediately, as it did previously when INET support was absent.

[pouria]

functionally LGTM and it will fix our compliance with RFC8925.
I'm not an rc expert. So I can't comment on its code.
But it works perfectly with D56637.

[zarychtam_plan-b.pwste.edu.pl]

Cache and reduce address-family support check as suggested.

[zarychtam_plan-b.pwste.edu.pl]

Improve route acquisition message when both default routes are added simultaneously; clarify AF checking comment; remove empty line.

Thanks for testing and feedback, including direct outreach.

While working on this change, I noticed that devd(8) requires INET to function; report and fix submitted (PR 295045).