RFR: 8264744: (fs) Use file cloning in Linux and macOS versions of FileChannel transfer and Files copy methods [v2]

[Brian Burkhalter]

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

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

  8264744: Refactor into provider+dispatcher to reduce amount of JNI code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9486/files
  - new: https://git.openjdk.org/jdk/pull/9486/files/35ed61e2..d2a862c4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9486&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9486&range=00-01

  Stats: 336 lines in 10 files changed: 195 ins; 125 del; 16 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