[jdk13u-dev] RFR: 8278851: Correct signer logic for jars signed with multiple digest algorithms

[Yuri Nesterenko]

I'd like to backport it for parity with major releases.
Version from 15u  applied with only old copyright change in one file. All security tests run OK.

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Commit messages:
 - Backport 61b8944327e3d12cf58dc3f6bc45ecbeba4ef611

Changes: https://git.openjdk.java.net/jdk13u-dev/pull/345/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk13u-dev&pr=345&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278851
  Stats: 272 lines in 4 files changed: 221 ins; 19 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk13u-dev/pull/345.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk13u-dev pull/345/head:pull/345

PR: https://git.openjdk.java.net/jdk13u-dev/pull/345