Allows to get int value for a given command line argument.
Returns the value associated with the argument name. The returned may be 0 which means the user may not defined that argument or may defined it by using the 0 as value. To avoid confusing cases you should use exarg_is_defined to know if the user have used that argument and then call this function.
- the value defined for this argument. If the argument is null or the exarg is not initialized the function will return -1.