RFR: 8306632: Add a JDK Property for specifying DTD support

[Joe Wang]

Add a JDK Impl specific property 'jdk.xml.dtd.support' for applications to specify how DTDs are handled. This property is uniformly supported across the JDK XML libraries. It complements, rather than replaces, the existing properties that are supportDTD for StAX and disallow-doctype-decl for DOM and SAX processors, which means the later will continue to work as they were and that if they are set, the new property will have no effect. Applications that use the existing properties continue to work as expected.

   This patch continues the path made previously with Xalan and XPath in which functions were moved into the jdk/xml classes. Similar changes are now made with the Xerces and XML classes, consolidating functions into the jdk/xml classes, reducing duplicate code for easier future maintenance.

Tests: new tests are added to cover the various processors.
       Existing tests pass. Only one was updated due to the change to the property impl.

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Commit messages:
 - fix Whitespace errors
 - 8306632: Add a JDK Property for specifying DTD support

Changes: https://git.openjdk.org/jdk/pull/15075/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15075&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306632
  Stats: 3337 lines in 71 files changed: 2019 ins; 1135 del; 183 mod
  Patch: https://git.openjdk.org/jdk/pull/15075.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15075/head:pull/15075

PR: https://git.openjdk.org/jdk/pull/15075