RFR: 8259801: Enable XML Signature secure validation mode by default [v2]

[Sean Mullan]

> This change enables the XML Signature secure validation mode by default. This will improve out of the box security by restricting signatures that contain potentially unsafe content by default.
> 
> Please also review the CSR: https://bugs.openjdk.java.net/browse/JDK-8260154

Sean Mullan 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
 - Minor re-wording of implNote.
 - Initial fix.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2197/files
  - new: https://git.openjdk.java.net/jdk/pull/2197/files/0c2e0498..ff15a1d4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2197&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2197&range=00-01

  Stats: 37355 lines in 801 files changed: 14132 ins; 17886 del; 5337 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2197.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2197/head:pull/2197

PR: https://git.openjdk.java.net/jdk/pull/2197