net/dns: only attempt registry changes on address families that are actually enabled on interface

We stop waiting to see if registry keys come available; this causes bad
interactions with the LocalBackend watchdog.

Instead we check the network interface for availability of AF_INET,
AF_INET6, and AF_NETBIOS. If no IP families are available on the
interface, we fail with an error. Otherwise we only attempt to configure
DNS for the address families that are actually enabled.

We also avoid changing any NetBIOS settings if AF_NETBIOS is disabled.

Fixes #46276

Change-Id: Ic7ae8a3dc810f4c428085f8b3ad905c6c32ae351
Signed-off-by: Aaron Klotz <aaron@tailscale.com>