RFR: 8264744: (fs) Use file cloning in Linux and macOS versions of FileChannel transfer and Files copy methods [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - 8264744: Refactor into UnixCopyFile hierarchy
 - Merge
 - 8264744: Address review comments aside from refactoring
 - 8264744: Refactor into provider+dispatcher to reduce amount of JNI code
 - 8264744: (fc, fs) Support file cloning in Unix versions of FileChannel transfer and Files copy methods

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9486/files
  - new: https://git.openjdk.org/jdk/pull/9486/files/6f3e4517..079dda1b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9486&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9486&range=02-03

  Stats: 21968 lines in 777 files changed: 13834 ins; 5618 del; 2516 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