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

[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.

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Commit messages:
 - 8269074: (fs) Files.copy fails to copy from /proc on some linux kernel versions

Changes: https://git.openjdk.java.net/jdk/pull/4543/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4543&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269074
  Stats: 41 lines in 1 file changed: 22 ins; 17 del; 2 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