RFR: 8293121: (fs) Refactor UnixFileSystem copying into generic Unix, Linux, and BSD implementations [v4]

[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: Address review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10093/files
  - new: https://git.openjdk.org/jdk/pull/10093/files/be36c5a6..b6cb9490

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=02-03

  Stats: 101 lines in 5 files changed: 7 ins; 76 del; 18 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