[jdk8u-dev] RFR: 8278851: Correct signer logic for jars signed with multiple digest algorithms

[Severin Gehwolf]

Please review this backport which fixes a regression new in 8u362 due to [JDK-8269039](https://bugs.openjdk.org/browse/JDK-8269039). For some jars which are signed with an obsolete digest algorithm, after JDK-8269039, they're treated as unsigned even though one of the signatories used a valid (non-obsolete) digest algo.

The backport is not clean, mainly because of JDK-8275887 (which in 11u wasn't present when that bug was backported), copyright headers and different test infra in 8u. So the test looks a bit different so that it works on 8u. All of the changes are fairly trivial to resolve. Changes in `SecurityUtils.java` aren't needed as 8u-dev already has them.

Testing:
 - [x] `jdk/test/sun/security/tools/jarsigner` tests pass.
 - [x] new regression test. Fails prior product fix, passes after.

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Commit messages:
 - Backport cbe497394786ff76a09f9743040e3ba96ee8298f

Changes: https://git.openjdk.org/jdk8u-dev/pull/282/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=282&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278851
  Stats: 264 lines in 3 files changed: 215 ins; 19 del; 30 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/282.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/282/head:pull/282

PR: https://git.openjdk.org/jdk8u-dev/pull/282