RFR: 8266589: (fs) Improve performance of Files.copy() on macOS using copyfile(3)

[Brian Burkhalter]

Please consider this request to use `fcopyfile(3)` to copy files via `java.nio.file.Files(Path,Path,CopyOption...)` on macOS. This change would improve the throughput of `Files.copy()` as shown in these results:

**Copy via 8192 byte buffers**


Benchmark           (size)   Mode  Cnt     Score     Error  Units
FilesCopy.copy       10240  thrpt    5  5432.671 ± 137.114  ops/s
FilesCopy.copy       51200  thrpt    5  3959.145 ± 157.467  ops/s
FilesCopy.copy      102400  thrpt    5  2931.325 ± 109.200  ops/s
FilesCopy.copy      512000  thrpt    5   655.550 ±  39.919  ops/s
FilesCopy.copy     1048568  thrpt    5   375.127 ±  10.111  ops/s
FilesCopy.copy    10485760  thrpt    5    64.048 ±   5.740  ops/s
FilesCopy.copy   104857600  thrpt    5     6.893 ±   0.415  ops/s
FilesCopy.copy  1073741824  thrpt    5     0.717 ±   0.026  ops/s


**Copy via fcopyfile**


Benchmark           (size)   Mode  Cnt     Score     Error  Units
FilesCopy.copy       10240  thrpt    5  5070.255 ± 817.419  ops/s
FilesCopy.copy       51200  thrpt    5  4469.218 ±  65.634  ops/s
FilesCopy.copy      102400  thrpt    5  3997.718 ± 301.440  ops/s
FilesCopy.copy      512000  thrpt    5  2064.223 ±  68.893  ops/s
FilesCopy.copy     1048568  thrpt    5   817.743 ±  83.076  ops/s
FilesCopy.copy    10485760  thrpt    5   145.799 ±   3.674  ops/s
FilesCopy.copy   104857600  thrpt    5    24.706 ±  14.178  ops/s
FilesCopy.copy  1073741824  thrpt    5     1.386 ±   0.073  ops/s


The smallest size tested shows a small degradation in throughput, but this could be rectified if desired by adding an empirically determined size threshold under which user-space buffers would be used instead of `fcopyfile()`.

A small change is also made to the Linux `sendfile()` portion to use the largest permitted `count` value if the copy is uninterruptible, i.e., `cancel == NULL`.

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Commit messages:
 - 8266589: (fs) Improve performance of Files.copy() on macOS using copyfile(3)

Changes: https://git.openjdk.java.net/jdk/pull/3890/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3890&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266589
  Stats: 38 lines in 1 file changed: 35 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3890.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3890/head:pull/3890

PR: https://git.openjdk.java.net/jdk/pull/3890