ipn/ipnserver: validate Host header on debug ServeHTMLStatus status

Updates tailscale/corp#7948

Change-Id: I3a8c64f353af1eeae620812b2700ce4af4fbbc88
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f