[proxy] by default, enable zero-copy when the protocol handler says it can
[proxy] by default, enable zero-copy when the protocol handler says it can

devnexen/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,yannick/h2o