net/dns, health: raise health warning for failing forwarded DNS queries (#12888)

updates tailscale/corp#21823

Misconfigured, broken, or blocked DNS will often present as
"internet is broken'" to the end user.  This  plumbs the health tracker
into the dns manager and forwarder and adds a health warning
with a 5 second delay that is raised on failures in the forwarder and
lowered on successes.

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

net/dns/manager.go

@@ -82,7 +82,7 @@ func NewManager(logf logger.Logf, oscfg OSConfigurator, health *health.Tracker,
 
 	m := &Manager{
 		logf:     logf,
-		resolver: resolver.New(logf, linkSel, dialer, knobs),
+		resolver: resolver.New(logf, linkSel, dialer, health, knobs),
 		os:       oscfg,
 		health:   health,
 		knobs:    knobs,