RFR: 8274112: (fc) Tune FileChannel.transferTo() [v5]

[Brian Burkhalter]

> Please consider this patch which would improve the performance of
> `FileChannel.transferTo()` on macOS and Windows.

Brian Burkhalter has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - 8274112: Remove native read-write copy; add threshold for mapped copy; use Linux copy_file_range() for direct copy if available
 - Merge
 - 8274112: Make transferTo0() static; remove check for null FileDescriptor
 - 8274112: Modify transfer_read_write() not to throw if read or write fails
 - 8274112: Refactor to use transferToFileChannel()
 - 8274112: (fc) Tune FileChannel.transferTo()

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5623/files
  - new: https://git.openjdk.java.net/jdk/pull/5623/files/001ffa2f..65e3adca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5623&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5623&range=03-04

  Stats: 1488877 lines in 8086 files changed: 841819 ins; 601486 del; 45572 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5623.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5623/head:pull/5623

PR: https://git.openjdk.java.net/jdk/pull/5623