RFR: 6980847: (fs) Files.copy needs to be tuned [v4]

[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: Impose minimum transfer size of 16384; add posix_fadvise() on Linux

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9161/files
  - new: https://git.openjdk.org/jdk/pull/9161/files/bc4096c6..0a77bd26

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9161&range=02-03

  Stats: 20 lines in 2 files changed: 14 ins; 0 del; 6 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