RFR: 8341775: Duplicate manifest files are removed by jarsigner after signing

[Kevin Driver]

JDK-8341775: In the case where there is a *single* META-INF directory but potentially *multiple* manifest files of different cases, print a warning before selecting the first one and ignoring the rest (the current behavior should be maintained).

**Note**: We cannot (so far) pass whether the verbose flag is set to the class that does this processing. We may want to add a property to the builder for this. As-is, the message will be printed via `System.err` whether verbose is set or not.

-------------

Commit messages:
 - JDK-8341775: In the case where there is a *single* META-INF directory but potentially *multiple* manifest files of different cases, print a warning before selecting the first one and ignoring the rest.

Changes: https://git.openjdk.org/jdk/pull/22222/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22222&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341775
  Stats: 26 lines in 1 file changed: 17 ins; 8 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22222.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22222/head:pull/22222

PR: https://git.openjdk.org/jdk/pull/22222