RFR: 8114830: (fs) Files.copy fails due to interference from something else changing the file system

[Brian Burkhalter]

Throw a `FileSystemException` if attempting to create the target file with `O_EXCL` fails with `EEXIST`.

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Commit messages:
 - 8114830: (fs) Files.copy fails due to interference from something else changing the file system

Changes: https://git.openjdk.org/jdk/pull/15141/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8114830
  Stats: 117 lines in 2 files changed: 117 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15141.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15141/head:pull/15141

PR: https://git.openjdk.org/jdk/pull/15141