MFC: Fix local-unbound setup for some IPv6 deployments.

On behalf of: des
PR: 250984
Sponsored by: Rubicon Communications, LLC ("Netgate")
Approved by: re (cperciva)

(cherry picked from commit e5f02c140bf1e519a95bd6331382e8a2a1b6219f)
(cherry picked from commit 34e12e9a177a1d2386a855eb5a497f6f8a680180)