Backpack v1.6.1

A hotfix. Tunnels updated to v1.6.0 came up, held a good ping, and then carried
nothing: sites and applications would not open through them.

Fixed

• Forwarded connections stopped carrying data. v1.6.0 added a zero-copy path
  to the forwarded relay — between two ordinary sockets, the kernel moved the
  bytes itself instead of them passing through this process. It is reverted. The
  relay is byte-for-byte the loop the upstream project uses, which is the
  behaviour proven on real tunnels; the optimisation was worth CPU on the plain
  TCP transport and nothing else, and it was the only place the forwarded data
  path had diverged. The traffic counting that existed only to serve it is gone
  with it, back to counting each read and write.

  Speed is unaffected: the ceiling is set by the mux stream window and the KCP
  window, which this never touched. Every performance preset behaves exactly as
  it did.

• A connection limit leaked a slot on every timeout. A forwarded connection
  that waited more than three seconds for a tunnel connection to pair with was
  closed without releasing the slot it took when it was accepted — only the
  handler frees that, and the handler never runs for a connection that timed
  out. On a tunnel with max_connections set, the limit filled up permanently
  until it would accept nothing at all. Tunnels with no limit configured were
  never affected, because the limiter does not count at all when it is unlimited.

Notes

No config, wire protocol, or tunnel behaviour changes. Updating replaces the
binary.