ipn/ipnlocal: add health warning for Tailscale SSH + SELinux

Updates #4908

Change-Id: If46be5045b13dd5c3068c334642f89b5917ec861
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f