RFR: 8293121: (fs) Refactor UnixFileSystem copying into generic Unix, Linux, and BSD implementations [v2]

[Brian Burkhalter]

> Refactor `UnixFileSystem` and related classes to move implementations specific to Linux and BSD (macOS) into OS-specific classes and native code. Generic Unix implementations remain in existing `Unix*` files.

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

  8293121: Absorb UnixCopyFile into UnixFileSystem; remove UnixCopyFile hierarchy

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10093/files
  - new: https://git.openjdk.org/jdk/pull/10093/files/dd1f7bcb..84a84ba0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=00-01

  Stats: 1687 lines in 8 files changed: 770 ins; 909 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/10093.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10093/head:pull/10093

PR: https://git.openjdk.org/jdk/pull/10093