RFR: 8291067: macOS should use O_CLOEXEC instead of FD_CLOEXEC [v2]

[Gerard Ziemski]

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

Gerard Ziemski has updated the pull request incrementally with two additional commits since the last revision:

 - add test to verify that open(O_CLOEXEC) implies FD_CLOEXEC on macOS
 - assume O_CLOEXEC on macOS

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9663/files
  - new: https://git.openjdk.org/jdk/pull/9663/files/7628e09a..82d1bbad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9663&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9663&range=00-01

  Stats: 33 lines in 2 files changed: 10 ins; 20 del; 3 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