RFR: 8267820: (fs) Files.copy should attempt to copy POSIX attributes when target file in custom file system

[Brian Burkhalter]

In `java.nio.file.Files.copy()` copy POSIX attributes even if the target is from a different file system provided both the source and target file systems support such attributes.

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Commit messages:
 - 8267820: (fs) Files.copy should attempt to copy POSIX attributes when target file in custom file system

Changes: https://git.openjdk.java.net/jdk/pull/7627/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7627&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267820
  Stats: 68 lines in 5 files changed: 41 ins; 9 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7627.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7627/head:pull/7627

PR: https://git.openjdk.java.net/jdk/pull/7627