RFR: 8316964: Security tools should not call System.exit [v4]

[Weijun Wang]

> Remove most `System.exit()` calls in various security tools and only leave one in the `main` method. This paves the way to convert them to JSR 199 tools.
> 
> Note: before this change, the behavior of a successful `main()` method execution is not consistent. Sometimes the method returns silently, sometimes a `System.exit(0)` is called somewhere. After this change, the method always returns silently. This makes sure that existing programs that calls the `main` method directly will continue and does not exit the VM.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  displayError

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15951/files
  - new: https://git.openjdk.org/jdk/pull/15951/files/e22257a9..ffb17d0b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15951&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15951&range=02-03

  Stats: 6 lines in 1 file changed: 1 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15951/head:pull/15951

PR: https://git.openjdk.org/jdk/pull/15951