[jdk8u-dev] RFR: 8087283: Add support for the XML Signature here() function to the JDK XPath implementation

[Yuri Nesterenko]

I'd like to backport this jdk9 change to jdk8 because it would make backporting future important fixes and related tests easier. It is marked as Enhancement, however with time the absence of it would become nuisance. 
I've tested it so far with various XML-related tests. Tier tests are running.

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Commit messages:
 - Backport 22fad64529a890dd3ae8b07c7981d9a720cf8e96

Changes: https://git.openjdk.java.net/jdk8u-dev/pull/67/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk8u-dev&pr=67&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8087283
  Stats: 135 lines in 3 files changed: 120 ins; 1 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk8u-dev/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk8u-dev pull/67/head:pull/67

PR: https://git.openjdk.java.net/jdk8u-dev/pull/67