shell run in backgroup to keep gtm master of postgres-xl alive

run on gtm master and gtm slave
    # gcc -O3 -Wall -Wextra -Werror -g -o port_probe ./port_probe.c
    # chmod 555 port_probe
    # mv port_probe /usr/local/bin
    //test port_probe:
    # port_probe $node_ip $port
    # echo "su - postgres -c /home/postgres/keep_gtm_alive.sh &" >> /etc/rc.local
    # chmod +x /etc/rc.d/rc.local
    # init 6

after the machine reboot, the script will run in backgroup to keep gtm master of postgres-xl alive if the gtm slave is ok.