RFR: 8269074: (fs) Files.copy fails to copy from /proc on some linux kernel versions [v2]

[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: Move user-space copy to separate function

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4543/files
  - new: https://git.openjdk.java.net/jdk/pull/4543/files/4d6c7c34..a88c3a7b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4543&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4543&range=00-01

  Stats: 54 lines in 1 file changed: 32 ins; 19 del; 3 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