[jdk18u] RFR: 8278851: Correct signer logic for jars signed with multiple digestalgs

[Sean Coffey]

Clean port to jdk18u. Awaiting 18u integration approval.

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Commit messages:
 - Backport 61b8944327e3d12cf58dc3f6bc45ecbeba4ef611

Changes: https://git.openjdk.java.net/jdk18u/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18u&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278851
  Stats: 264 lines in 3 files changed: 214 ins; 20 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk18u/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18u pull/19/head:pull/19

PR: https://git.openjdk.java.net/jdk18u/pull/19