Varnish 4

This is working well for us so far!

Also, [this concurrency issue](http://stackoverflow.com/questions/22470422/hiredis-timeout-with-large-number-of-concurrent-requests) seems to be resolved in this build.  Can't say if it's due to the changes reflected here, or in Varnish 4, or in libhiredis, but I can no longer reproduce it with my build, using the Varnish master ( varnish/varnish-cache@4354e5e04a6af43394bbbdb32dc2654ca4887de8 ) and the Ubuntu-packaged `libhiredis-dev` which should be redis/hiredis@0fff0f182b96b4ffeee8379f29ed5129c3f72cf7

Note: I replaced `WS_Dup` with `WS_Printf`, but it is possible that `WS_Copy` would be a better choice.  C is not my strong suit, and I'd be interested to hear your opinion on that.