cmd/tailscaled: default to userspace-networking on plan9

No tun support yet.

Updates #5794

Change-Id: Ibd8db67594d4c65b47e352ae2af2ab3d2712dfad
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