RFR: 8293122: (fs) Use file cloning in macOS version of Files::copy method [v4]

[Brian Burkhalter]

> Use BSD system call `clonefile` if possible when using `Files::copy` on macOS.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8293122: Move rollback unlink to copyFile method

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10188/files
  - new: https://git.openjdk.org/jdk/pull/10188/files/f51cd186..96db8562

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10188&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10188&range=02-03

  Stats: 24 lines in 1 file changed: 9 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10188.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10188/head:pull/10188

PR: https://git.openjdk.org/jdk/pull/10188