The tunnel acts like a client and a server and forwards bytes between
two parties. It can then pause in the middle of a session and control
how much data can go in either direction. Combined with barriers it can
be used to trigger socket timeouts, possibly in the middle of protocol
frames.

A tunnel works with two threads: one to execute its spec and one to poll
both parties and transfer data accordingly. The two threads synchronize
via basic pthread primitives and follow a very simplistic state machine:

    ACCEPT -> RUNNING <-> PAUSED -> SPEC DONE -> POLL DONE -> STOPPED

Despite being simplistic, the coordination of two threads would likely
complicate the implementation of a `-repeat` action, but it is still
possible to `start` a tunnel again as shown in the c106 test case that
exercises basic coverage involving a varnish instance.

Usage is documented in the vtc(7) manual.