RFR: 8257722: Improve "keytool -printcert -jarfile" output

[Hai-May Chao]

This change does a few improvements to the output of `keytool -printcert -jarfile` command to help readability and diagnosis.

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Commit messages:
 - 8257722: Improve "keytool -printcert -jarfile" output

Changes: https://git.openjdk.java.net/jdk/pull/6126/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6126&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257722
  Stats: 81 lines in 3 files changed: 75 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6126.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6126/head:pull/6126

PR: https://git.openjdk.java.net/jdk/pull/6126