RFR: 8293121: (fs) Refactor UnixFileSystem copying into generic Unix, Linux, and BSD implementations [v3]

[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: Forgot to move UnixCopyFile.c to UnixFileSystem.c

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10093/files
  - new: https://git.openjdk.org/jdk/pull/10093/files/84a84ba0..be36c5a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10093&range=01-02

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 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