RFR: 6980847: (fs) Files.copy needs to be tuned [v8]

[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 one additional commit since the last revision:

  6980847: lcm IAE -> assert; nested try-with-resources; bufferCopy0 -> bufferedCopy0

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9161/files
  - new: https://git.openjdk.org/jdk/pull/9161/files/04d75022..6c7ac10c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=06-07

  Stats: 16 lines in 1 file changed: 5 ins; 3 del; 8 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