Updating configTest to include shutdown port validation

[Amit Pande]

Hello,

I was testing out the "configtest"  option of the catalina.sh/.bat and observed 
that does not do validation for the shutdown port.

https://github.com/apache/tomcat/blob/8bbdabe02c384cf15aa43dafb66a5b42440edac3/java/org/apache/catalina/startup/Bootstrap.java

                case "configtest":
                    daemon.load(args);
                    if (null == daemon.getServer()) {
                        System.exit(1);
                    }
                    // Can we add the shutdown port validation code here?
                   // Check if the port is an integter, check if this port is 
available for use
                  // Not 100% sure yet if this is the right place
                   validateShutdownPort(daemon.getServer().getPorrt());
                    System.exit(0);
                    break;

Appreciate feedback on whether this is a good idea to enhance the server.xml 
config validation check?

Thanks,
Amit