net/dns: cache dns.Config for reuse when compileConfig fails (#16059)

fixes tailscale/corp#25612

We now keep track of any dns configurations which we could not
compile. This gives RecompileDNSConfig a configuration to
attempt to recompile and apply when the OS pokes us to indicate
that the interface dns servers have changed/updated.   The manager config
will remain unset until we have the required information to compile
it correctly which should eliminate the problematic SERVFAIL
responses (especially on macOS 15).

This also removes the missingUpstreamRecovery func in the forwarder
which is no longer required now that we have proper error handling
and recovery manager and the client.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

net/dns/resolver/tsdns.go

@@ -251,15 +251,6 @@ func New(logf logger.Logf, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, h
 	return r
 }
 
-// SetMissingUpstreamRecovery sets a callback to be called upon encountering
-// a SERVFAIL due to missing upstream resolvers.
-//
-// This call should only happen before the resolver is used. It is not safe
-// for concurrent use.
-func (r *Resolver) SetMissingUpstreamRecovery(f func()) {
-	r.forwarder.missingUpstreamRecovery = f
-}
-
 func (r *Resolver) TestOnlySetHook(hook func(Config)) { r.saveConfigForTests = hook }
 
 func (r *Resolver) SetConfig(cfg Config) error {