cmd/tailscale/cli: add "configure jetkvm" subcommand

To write the init script.

And fix the JetKVM detection to work during early boot while the filesystem
and modules are still being loaded; it wasn't being detected on early boot
and then tailscaled was failing to start because it didn't know it was on JetKVM
and didn't modprobe tun.

Updates #16524

Change-Id: I0524ca3abd7ace68a69af96aab4175d32c07e116
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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