use iovec list to build prefix instead of char pointers
use iovec list to build prefix instead of char pointers

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