RFR: 8268613: jar --validate should check inital entries of a JAR file

[Christian Stein]

Please review this change to include a validation check for expecting an optional JAR manifest entry being the first or second entry in JAR file. As the second entry, it must be only preceeded by an entry for the `META-INF/` directory.

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Commit messages:
 - Compact implementation
 - Inline method
 - Manifest is optional, no need to add it here
 - 8268613: jar --validate should check inital entries of a JAR file

Changes: https://git.openjdk.org/jdk/pull/28154/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8268613
  Stats: 95 lines in 3 files changed: 95 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28154/head:pull/28154

PR: https://git.openjdk.org/jdk/pull/28154