Remove pool_log/pool_error calls from signal handlers.

Remove pool_log/pool_error calls from signal handlers.

pool_log/pool_error calls malloc(), which is not safe to be called
inside signal handlers, per discussion in [pgpool-hackers: 1950].  I
ifdef out them, rather than simply remove them in a hope we someday
find a better solution which make calling the functions inside signal
handlers.

Not that I did not touch exit_handler() of pgpool_main.c because
removing pool_log/pool_debug from them loses informative message like
"received smart shutdown request". Pgpool-II main process do not
heavily use malloc(), so the risk is minimum, I guess.