cmd/derper: add user timeout and reduce TCP keepalive

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The derper sends an in-protocol keepalive every 60-65s, so frequent TCP
keepalives are unnecessary. In this tuning TCP keepalives should never
occur for a DERP client connection, as they will send an L7 keepalive
often enough to always reset the TCP keepalive timer. If however a
connection does not receive an ACK promptly it will now be shutdown,
which happens sooner than it would with a normal TCP keepalive tuning.

This re-tuning reduces the frequency of network traffic from derp to
client, reducing battery cost.

Updates tailscale/corp#17587
Updates #3363

Signed-off-by: James Tucker <james@tailscale.com>

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