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

[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.

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Commit messages:
 - 6980847: (fs) Files.copy needs to be tuned

Changes: https://git.openjdk.org/jdk/pull/9161/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6980847
  Stats: 94 lines in 4 files changed: 79 ins; 0 del; 15 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