If the client uses the default Tailscale control URL, validate that all
PopBrowserURLs are under tailscale.com or *.tailscale.com. This reduces
the risk of a compromised control plane opening phishing pages for
example.
The client trusts control for many other things, but this is one easy
way to reduce that trust a bit.
Fixes#11393
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>