[crac] RFR: 8352413: [CRaC] crexec fails to pass some options when CRAC_CRIU_OPTS is already used

[Timofei Pushkin]

This contains the change from #216 so that should be merged first.

The fix itself is small but coming up with a way to test it was not trivial:
1. I've split `jdk/crac/CracEngineOptionsTest.java` onto `jdk/crac/engineOptions/ParsingTest.java` and `jdk/crac/engineOptions/HelpTest.java` because it was getting too large (nothing added/removed, just split).
2. Added `jdk/crac/engineOptions/CracCriuOptsTest.java` to regression-test the main fix of this PR (this test depends on #216).
3. Removed a part that tested that `args` are actually applied by `crexec` from `jdk/crac/VMOptionsTest.java` because (2) is now effectively tests this (`VMOptionsTest` wasn't a proper place for this to begin with, it just was convenient).

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Commit messages:
 - Fix CRAC_CRIU_OPTS appending
 - Direct map by default

Changes: https://git.openjdk.org/crac/pull/217/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=217&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352413
  Stats: 618 lines in 6 files changed: 369 ins; 232 del; 17 mod
  Patch: https://git.openjdk.org/crac/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/217/head:pull/217

PR: https://git.openjdk.org/crac/pull/217