A new mode, backend mode, in varnishncsa is introduced. It lets you
log trafic from varnish to the backends. The normal (default) mode is
now named "client mode" and can be explicitly selected using -c. You
can run varnishncsa in backend mode in addition to varnishncsa in
client mode, but it is also possible to run in mixed mode by
specifying both -b and -c.

New formatter added: %{Varnish:side}x will be either "b" or "c",
depending on where the request was made.