minor: disable external dtd load by default

[romani]

no system property result in:

java.lang.NullPointerException
    at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDProcessor.endDTD(XMLDTDProcessor.java:1349)
    at com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.endEntity(XMLDTDScannerImpl.java:652)
    at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.endEntity(XMLEntityManager.java:1401)
    at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.load(XMLEntityScanner.java:1917)
    at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.skipSpaces(XMLEntityScanner.java:1655)
    at com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.skipSeparator(XMLDTDScannerImpl.java:2082)
    at com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.scanDecls(XMLDTDScannerImpl.java:2058)
    at com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.scanDTDExternalSubset(XMLDTDScannerImpl.java:301)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.dispatch(XMLDocumentScannerImpl.java:1175)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.next(XMLDocumentScannerImpl.java:1045)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:933)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:602)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:505)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:841)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:770)
    at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
    at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
    at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:643)
    at com.puppycrawl.tools.checkstyle.XmlLoader.parseInputSource(XmlLoader.java:84)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoader.parseInputSource(ConfigurationLoader.java:197)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoader.loadConfiguration(ConfigurationLoader.java:443)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoader.loadConfiguration(ConfigurationLoader.java:395)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoader.loadConfiguration(ConfigurationLoader.java:372)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoader.loadConfiguration(ConfigurationLoader.java:209)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoaderTest.loadConfiguration(ConfigurationLoaderTest.java:62)
    at com.puppycrawl.tools.checkstyle.ConfigurationLoaderTest.testExternalEntity(ConfigurationLoaderTest.java:410)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

[romani]

xdoc update and special issue creation (breaking compatibilty) will be done a bit later on

[rnveach]

@romani

no system property result in: java.lang.NullPointerException

Should we add a test to show this and make sure it is part of regression?
I only see tests specifying true and not false and an exception.

[rnveach]

extrnal => external

[romani]

fixed.

[romani]

Should we add a test to show this and make sure it is part of regression?

we never did a tests to show how checksyle might fail on misconfiguration, but I stored exception on purpose to let user easily google it.
I moved stacktrace to issue comment - #6474 .