[crac] RFR: Close extraneous file descriptors

[Radim Vansa]

Java code cannot use inherited non-standard file descriptors; these can cause trouble when trying to checkpoint the process via CRIU, and hence we will proactively close them on boot.

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Commit messages:
 - Remove unneeded check
 - Applied review suggestions
 - Close non-standard file descriptors on boot

Changes: https://git.openjdk.org/crac/pull/45/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=45&range=00
  Stats: 253 lines in 7 files changed: 237 ins; 12 del; 4 mod
  Patch: https://git.openjdk.org/crac/pull/45.diff
  Fetch: git fetch https://git.openjdk.org/crac pull/45/head:pull/45

PR: https://git.openjdk.org/crac/pull/45