RFR: 8291067: macOS should use O_CLOEXEC instead of FD_CLOEXEC

[Gerard Ziemski]

O_CLOEXEC is also available on macOS (__DARWIN_C_LEVEL >= 200809L), so use it same as on linux.

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Commit messages:
 - match existing code
 - add missing code from merge
 - Use O_CLOEXEC if available

Changes: https://git.openjdk.org/jdk/pull/9663/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9663&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291067
  Stats: 60 lines in 1 file changed: 36 ins; 22 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9663.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9663/head:pull/9663

PR: https://git.openjdk.org/jdk/pull/9663