control/controlclient: use slices package more

Updates #cleanup

Change-Id: Ic17384266dc59bc4e710efdda311d6e0719529da
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