RFR: 8271868: Warn user when using mac-sign option with unsigned app-image. [v2]

[Andy Herrick]

> 8271868: Warn user when using mac-sign option with unsigned app-image.

Andy Herrick has updated the pull request incrementally with one additional commit since the last revision:

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

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5004/files
  - new: https://git.openjdk.java.net/jdk/pull/5004/files/153e75ea..afc0f197

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5004&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5004&range=00-01

  Stats: 72 lines in 6 files changed: 30 ins; 27 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5004.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5004/head:pull/5004

PR: https://git.openjdk.java.net/jdk/pull/5004