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

[Brian Burkhalter]

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

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Commit messages:
 - 8293122: (fs) Use file cloning in macOS version of Files::copy method

Changes: https://git.openjdk.org/jdk/pull/10188/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10188&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293122
  Stats: 231 lines in 7 files changed: 229 ins; 0 del; 2 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