Fix health check process is not shutting down in certain cases.

Fix health check process is not shutting down in certain cases.

When watchdog detects fatal events, including not reaching to
trusted_servers, watchdog suicides with POOL_EXIT_FATAL exit status
code. In this case the parent of watchdog, the pgpool main process's
SIGCHILD handler reaper() exits and on_exit call back calls
system_will_go_down(), which in turn calls terminate_all_children().
Problem is, terminate_all_children() forgot to kill health check
process. This commit fixes that.

Also there are some not well behaving codings are enhanced.

Back patched to 3.7, when the bug was introduced.