Make 'prog' overrideable and more explicit

Both init.d scripts provided with Wildfly are fairly well designed because the JBOSS_USER, JBOSS_CONFIG and other variables can be override from outside, allowing people to integrate the server within their environment by just adding a script, overriding what needs to be, without changing the script delivered with the products.

However, for reason  unknown to me, the variable 'prog' , which name the service associated with Wildfly, can not be overriden. Those commits do the renaming and also rename the 'prog' variable into something more explicit and following the naming convention of the other overrideable vars.

Hope this helps !