Add new config parameter "log_pcp_processes",

Add new config parameter "log_pcp_processes",

This allows to disable logging about normal PCP Process fork and exit
status. When pcp command is executed, pgpool logs its fork/exit event
even if there's no error. This could fill up the pgpool log.

Abnormal fork/exit event will be logged even if the parameter is
disabled.

Author: Maximilien Cuony
Review and Japanese document by: Tatsuo Ishii