[crac] RFR: Collect CREngine child

[Anton Kozlov]

When VM calls for CREngine it does not properly awaits termination of the engine. In case of `criuengine`, this could lead for a dangling process in the CRIU image [1]. This patch adds missing waitpid that eliminates the problem.

[1] https://github.com/openjdk/crac/blob/crac/src/java.base/unix/native/criuengine/criuengine.c#L68

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Commit messages:
 - Collect CREngine child

Changes: https://git.openjdk.java.net/crac/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/?repo=crac&pr=18&range=00
  Stats: 16 lines in 1 file changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/crac/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/crac pull/18/head:pull/18

PR: https://git.openjdk.java.net/crac/pull/18