RFR: 8269074: (fs) Files.copy fails to copy from /proc on some linux kernel versions [v3]

[Brian Burkhalter]

> On Linux, if while executing `java.nio.file.Files.copy(Path,Path,CopyOption...)` the native function `sendfile(2)` fails with `EINVAL`, fall back to transferring via user-space buffers.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8269074: Minor restructuring

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4543/files
  - new: https://git.openjdk.java.net/jdk/pull/4543/files/a88c3a7b..f41ff6aa

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4543&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4543&range=01-02

  Stats: 79 lines in 1 file changed: 35 ins; 43 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4543.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4543/head:pull/4543

PR: https://git.openjdk.java.net/jdk/pull/4543