add child reaper and fix bug on http_handler_reset

forked child is waitpided and cleaned
It is tested that readret of read(pipe_fd) is enough to determine a finished cgi job.
There is a bug that prevents further pipelined POST request, though I fixed
without known exact buggy line, it seems to come from un-reset http_hander_t
members.

A key lesson is remember to clean up reusable object state!!!
A past lesson is to initialize all variables before any use!!!