RFR: 8259801: Enable XML Signature secure validation mode by default

[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

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Commit messages:
 - Minor re-wording of implNote.
 - Initial fix.

Changes: https://git.openjdk.java.net/jdk/pull/2197/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2197&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259801
  Stats: 55 lines in 3 files changed: 12 ins; 3 del; 40 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