Not yet used. This is being made available so magicsock/wgengine can
use it to ignore certain sends (UDP + DERP) later on at least mobile,
letting wireguard-go think it's doing its full attempt schedule, but
we can cut it short conditionally based on what we know from the
control plane.
Updates #7617
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia367cf6bd87b2aeedd3c6f4989528acdb6773ca7