mx: improve first usage experience: Create mx.ask_persist_env to handle env file modifications, make it deal with files missing final newline Use mx.ask_persist_env to persist the DEFAULT_VM Make mx_graal specify its version restrictions when asking for the default JDK When selecting versions manually or automatically, use JDKs from JAVA_HOME first, then from EXTRA_JAVA_HOMES, then from standard locations, then sort by version. Even if EXTRA_JAVA_HOMES is already defained, let the user decide which JAVA_HOME to use

public class ClasspathDump {
    public static void main(String[] args) {
        System.out.print(System.getProperty("sun.boot.class.path"));
        System.out.print("|");
        System.out.print(System.getProperty("java.ext.dirs"));
        System.out.print("|");
        System.out.print(System.getProperty("java.endorsed.dirs"));
    }
}