Fix hung up in pcp_exit_handler of pcp child main.

Fix hung up in pcp_exit_handler of pcp child main.

pcp_exit_handler is responsible for waiting for exiting pcp child
process. I suspect it is hung up in the wait loop in regression test
001.

https://www.pgpool.net/pipermail/pgpool-hackers/2023-September/004397.html

So I changed it so that it uses waitpid(2) with WNOHANG option.