Fix race condition between flag checking and signal blocking.

Fix race condition between flag checking and signal blocking.

Normally, this case solve to call pselect(2). But Linux's pselect(2)
is emulated by select(2) and sigprocmask(2). It contains this race.
So pgpool archieves using the self-pipe trick.

See 'man pselect' if you want to get more information.