net/dns: recheck DNS config on SERVFAIL errors (#12547)

Fixes tailscale/corp#20677

Replaces the original attempt to rectify this (by injecting a netMon
event) which was both heavy handed, and missed cases where the
netMon event was "minor".

On apple platforms, the fetching the interface's nameservers can
and does return an empty list in certain situations.   Apple's API
in particular is very limiting here.  The header hints at notifications
for dns changes which would let us react ahead of time, but it's all
private APIs.

To avoid remaining in the state where we end up with no
nameservers but we absolutely need them, we'll react
to a lack of upstream nameservers by attempting to re-query
the OS.

We'll rate limit this to space out the attempts.   It seems relatively
harmless to attempt a reconfig every 5 seconds (triggered
by an incoming query) if the network is in this broken state.

Missing nameservers might possibly be a persistent condition
(vs a transient error), but that would  also imply that something
out of our control is badly misconfigured.

Tested by randomly returning [] for the nameservers.   When switching
between Wifi networks, or cell->wifi, this will randomly trigger
the bug, and we appear to reliably heal the DNS state.

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

net/dns/resolver/tsdns.go

@@ -244,6 +244,13 @@ func New(logf logger.Logf, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, k
 	return r
 }
 
+// Called by the forwarder on SERVFAIL due to missing upstream resolvers
+// The func passed in here should attempt to re-query for those resolvers,
+// repair, or recover
+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 {