updates tailscale/corp#27145
We require a means to trigger a recompilation of the DNS configuration
to pick up new nameservers for platforms where we blend the interface
nameservers from the OS into our DNS config.
Notably, on Darwin, the only API we have at our disposal will, in rare instances,
return a transient error when querying the interface nameservers on a link change if
they have not been set when we get the AF_ROUTE messages for the link
update.
There's a corresponding change in corp for Darwin clients, to track
the interface namservers during NEPathMonitor events, and call this
when the nameservers change.
This will also fix the slightly more obscure bug of changing nameservers
while tailscaled is running. That change can now be reflected in
magicDNS without having to stop the client.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>
25d5f78c6e