When running on Home Assistant, use the X-Ingress-Path header to set the
URLPrefix that is passed to the frontend.
Also fix handling of errNotUsingTailscale in the auth handler
(previously it falling through to a later case and returning a 500).
Instead, it's just a terminal state with no auth needed.
Also disable SSH on Home Assistant, since it causes problems on startup
and doesn't make much sense anyway for that platform.
Updates #10261
Signed-off-by: Will Norris <will@tailscale.com>
5a2e6a6f7d