health: introduce captive-portal-detected Warnable (#12707)

Updates tailscale/tailscale#1634

This PR introduces a new `captive-portal-detected` Warnable which is set to an unhealthy state whenever a captive portal is detected on the local network, preventing Tailscale from connecting.



ipn/ipnlocal: fix captive portal loop shutdown


Change-Id: I7cafdbce68463a16260091bcec1741501a070c95

net/captivedetection: fix mutex misuse

ipn/ipnlocal: ensure that we don't fail to start the timer


Change-Id: I3e43fb19264d793e8707c5031c0898e48e3e7465

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

net/dnsfallback/dnsfallback_test.go

@@ -18,7 +18,7 @@ import (
 )
 
 func TestGetDERPMap(t *testing.T) {
-	dm := getDERPMap()
+	dm := GetDERPMap()
 	if dm == nil {
 		t.Fatal("nil")
 	}
@@ -78,7 +78,7 @@ func TestCache(t *testing.T) {
 	}
 
 	// Verify that our DERP map is merged with the cache.
-	dm := getDERPMap()
+	dm := GetDERPMap()
 	region, ok := dm.Regions[99]
 	if !ok {
 		t.Fatal("expected region 99")