Fix signal handler in pgpool main

I got a bug report regarding pgpool main process from Emond Papegaaij:
its exit/reload signal handler uses non signal safe functions, and it
could lead to a crash of the process.

Attached is the patch from him generated by Claude Code, with slight
modifications by me.

Basically it changes the signal handler to async-signal-safe calls
only: capture the signal number into a new volatile sig_atomic_t
main_exit_request, write one byte to the existing self-pipe to wake
the main loop, restore errno, and return. The actual shutdown is
performed synchronously by a new do_shutdown() function called from
the pgpol main loop at the top of every iteration (via
check_requests()) and also right after the inner pool_pause() returns,
so a signal arriving during the 2-second select() sleep is acted on
without an extra tick of latency.

I think this patch is important and worth to apply to all supported
branches.

However, since it touches one of the most critical part of pgpool, I
would like to apply to master branch first and see how our build-farm
acts before back patching (it will take a few days).

Comments?
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp