control/controlclient: scope a variable tighter, de-pointer a *time.Time

Just misc cleanups.

Updates #1909

Change-Id: I9d64cb6c46d634eb5fdf725c13a6c5e514e02e9a
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