RFR: 8264744: (fc, fs) Support file cloning in Unix versions of FileChannel transfer and Files copy methods

[Brian Burkhalter]

Add file cloning to `java.nio.channels.FileChannel::transferTo` and `java.nio.file.Files.copy(Path,Path)`.

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Commit messages:
 - 8264744: (fc, fs) Support file cloning in Unix versions of FileChannel transfer and Files copy methods

Changes: https://git.openjdk.org/jdk/pull/9486/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9486&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8264744
  Stats: 288 lines in 4 files changed: 279 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/9486.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9486/head:pull/9486

PR: https://git.openjdk.org/jdk/pull/9486