[8u] RFR Backport 8079693: Add support for ECDSA P-384 and P-521 curves to XML Signature

[Elliott Baron]

Hi,

I'd like to request a review to backport 8079693 to 8u.

This fix restores support for curves that were removed by the 
refactoring in "8046724: XML Signature ECKeyValue elements cannot be 
marshalled or unmarshalled". It also aids us in backporting "8177334: 
Update xmldsig implementation to Apache Santuario 2.1.1".

Original fix:
https://bugs.openjdk.java.net/browse/JDK-8079693
http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/5ad36a27ddf3

The original JDK 9 version of this fix applies cleanly, with one small 
exception:

test/javax/xml/crypto/dsig/GenerationTests:
- Bugs listed in jtreg tag differ.
-- "8046949: Generify the javax.xml.crypto API" not in 8u, due to API 
breakage.
-- Newer fixes "8074784: Additional tests for XML DSig API" and 
"8210736: jdk/javax/xml/crypto/dsig/GenerationTests.java slow on linux" 
already present in 8u.

8u webrev:
https://cr.openjdk.java.net/~ebaron/jdk8u/JDK-8079693/webrev.00/

Testing: x86_64 build, jdk_tier1, jdk_security tests

Thanks,
Elliott