RFR: 6980847: (fs) Files.copy needs to be tuned [v6]

[Brian Burkhalter]

> Modify `UnixCopyFile.copyFile()` to set the transfer size to the least common multiple of the source and destination file block sizes if the block sizes are not equal. Modify `WindowsFileCopy.copy()` to set `COPY_FILE_NO_BUFFERING` in the flags passed to `CopyFileEx()` if the size of the transfer is greater than a threshold.

Brian Burkhalter has updated the pull request incrementally with two additional commits since the last revision:

 - 6980847: Convert to fallback to copy0() method
 - 6980847: work in progress

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9161/files
  - new: https://git.openjdk.org/jdk/pull/9161/files/c30a6f0b..a6575484

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=04-05

  Stats: 122 lines in 4 files changed: 49 ins; 30 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/9161.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9161/head:pull/9161

PR: https://git.openjdk.org/jdk/pull/9161