Was done as part of e6fbc0cd54906c85f58f8d69fb5872886db8811d for ssh
work, but wasn't committed yet. Including it here both to minimize the
ssh diff size, and because I need it for a separate change.
Change-Id: If6eb54a2ca7150ace96488ed14582c2c05ca3422
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>