Close listen socket when smart shutdown request is made.

When smart shutdown process starts, pgpool children still listen on
the port and clients can send further connection requests which fail
in the end. Which is not only waste of time, but also prevents a load
balancer which sits in front of pgpool from realizing the pgpool is
going down.

Problem analyzed and patch provided by Junegunn Choi in pgpool-hackers
474, and enhanced to take care not only inet domain socket but UNIX
domain socket by me.
Add cautions that recovery commands are killed by statement_timeout of PostgreSQL.

recovery_1st_stage_command can be killed if its execution time is
longer than statement_timeout and online recovery fails. The problem
was found and reported by Sean Hogan at [pgpool-general:
2680]. Samething can be said to recovery_2nd_stage_command.