[jdk17] RFR: 8271868: Warn user when using mac-sign option with unsigned app-image.

[Andy Herrick]

This is a backport from JDK-18

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Commit messages:
 -     JDK-8271868: Warn user when using mac-sign option with unsigned app-image.

Changes: https://git.openjdk.java.net/jdk17/pull/305/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=305&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271868
  Stats: 65 lines in 9 files changed: 46 ins; 11 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/305.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/305/head:pull/305

PR: https://git.openjdk.java.net/jdk17/pull/305