[crac] RFR: Collect CREngine child [v2]

[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

Anton Kozlov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Check fork error
 - Merge remote-tracking branch 'jdk/crac/crac' into crengine-child
 - Collect CREngine child

-------------

Changes:
  - all: https://git.openjdk.java.net/crac/pull/18/files
  - new: https://git.openjdk.java.net/crac/pull/18/files/984c58f6..923d1090

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=crac&pr=18&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=crac&pr=18&range=00-01

  Stats: 247 lines in 10 files changed: 194 ins; 20 del; 33 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