Close listening socokets before forking

Currently when pgpool main process forks sub processes (child process
- user session process, pcp_main process, health check process,
streaming replication check process and life check process), they
inherits pgpool and pcp listening sockets. However some of processes
do not need those sockets:

- child process - pcp sockets are unnecessary
- pcp main process - pgpool sockets are unnecessary
- health check, streaming replication check and life check process -
pgpool and pcp sockets are unnecessary

It could be potential problem when those process go down. Since they
may keep the listening sockets for a while, which prevents next pgpool
starting up from binding those ports.

Attached patch closes those unnecessary sockets after forking. For
this purpose new function close_listening_sockets() is introduced.

Best regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp