[crac] RFR: 8361090: [CRaC] Make dev engines less confusing

[Timofei Pushkin]

- Makes `pauseengine` Linux-only since it is not fully implemented on MacOS and Windows
   - Also moves `criuengine` sources from `unix` to `linux` directory since it only supports Linux anyway
- Makes `simengine` and `pausengine` report errors and status in more details

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Commit messages:
 - Make dev engines less confusing

Changes: https://git.openjdk.org/crac/pull/239/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361090
  Stats: 250 lines in 7 files changed: 120 ins; 125 del; 5 mod
  Patch: https://git.openjdk.org/crac/pull/239.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/239/head:pull/239

PR: https://git.openjdk.org/crac/pull/239