cmd/derper: add user timeout and reduce TCP keepalive

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>

cmd/derper/depaware.txt

@@ -95,6 +95,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         tailscale.com/net/dnscache                                   from tailscale.com/derp/derphttp
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+
      💣 tailscale.com/net/interfaces                                 from tailscale.com/net/netmon+
+        tailscale.com/net/ktimeout                                   from tailscale.com/cmd/derper
         tailscale.com/net/netaddr                                    from tailscale.com/ipn+
         tailscale.com/net/netknob                                    from tailscale.com/net/netns
         tailscale.com/net/netmon                                     from tailscale.com/derp/derphttp+