fix for jpms by remove use of ResourceBundle.Control

[aanno]

I observed the following

1. The use of ResourceBundle.Control with the Java Platform Module System (jpms) is not possible. In Java >=9 the following method is present in ResourceBundle:

    private static void checkNamedModule(Class<?> caller) {
        if (caller.getModule().isNamed()) {
            throw new UnsupportedOperationException("ResourceBundle.Control not supported in named modules");
        }
    }

2. ResourceBundle *.properties files in epubcheck are UTF-8 encoded.

Hence this pull request will (a) eliminate the use of ResourceBundle.Control and unify the loading of *.properties files to enhance the compatibility of epubcheck with jpms.

Review und comments are appreciated.

Kind regards,

aanno