Fix for 0000296: PGPool v3.6.2 terminated by systemd because the service Type

Fix for 0000296: PGPool v3.6.2 terminated by systemd because the service Type
has been set to 'forking'

Removing the "-n" value assigned to OPTS variable in pgpool.sysconfig.
The problem was the systemd service with Type=forking expects the parent process
to exit after the startup is complete, but because the -n command line option
disables the daemon mode and systemd keeps on waiting for the Pgpool-II's parent
process to exit after startup, which never happens and eventually systemd
terminate the Pgpool-II after timeout.
As part of this commit I have also added a new variable STOP_OPTS which is
passed to ExecStop and can be used to pass extra command line options to
Pgpool-II stop command.