RFR: 8306632: Add a JDK Property for specifying DTD support [v2]

[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.

Joe Wang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into JDK-8306632
   Sync up to re-run tests.
 - fix Whitespace errors
 - 8306632: Add a JDK Property for specifying DTD support

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15075/files
  - new: https://git.openjdk.org/jdk/pull/15075/files/ce6cf352..b9877b15

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15075&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15075&range=00-01

  Stats: 68159 lines in 2227 files changed: 38336 ins; 14910 del; 14913 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