h2olog: add a reusable usage() function
h2olog: add a reusable usage() function

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