RFR: 8265175: (fs) Files.copy(Path, Path, CopyOption...) should use zero-copy on Linux

[Brian Burkhalter]

Please consider this request to change the underlying native implementation of `java.nio.file.Files.copy(Path,Path,CopyOption...)` on Linux only to perform zero-copy via `sendfile(2)`. The `sendfile()` system call is already used in `java.nio.channels.FIleChannel.transferTo​(long,long,WritableByteChannel)`. It is intentionally _not_ proposed to use `sendfile()` in the native macOS implementation as on macOS the function requires that the destination file descriptor is for a socket.

This change showed some performance improvement as measured by JMH.

**Before: user-space buffers (read() + write())**

Benchmark           (size)   Mode  Cnt      Score     Error  Units
FilesCopy.copy       10240  thrpt    5  39167.400 ± 683.887  ops/s
FilesCopy.copy       51200  thrpt    5  20782.622 ± 558.031  ops/s
FilesCopy.copy      102400  thrpt    5  13260.709 ± 176.673  ops/s
FilesCopy.copy      512000  thrpt    5   3171.837 ± 175.803  ops/s
FilesCopy.copy     1048568  thrpt    5   1654.253 ±  39.419  ops/s
FilesCopy.copy    10485760  thrpt    5    145.328 ±   7.192  ops/s
FilesCopy.copy   104857600  thrpt    5     12.440 ±   2.275  ops/s
FilesCopy.copy  1073741824  thrpt    5      1.073 ±   0.081  ops/s

**After: zero-copy (sendfile())**

Benchmark           (size)   Mode  Cnt      Score     Error  Units
FilesCopy.copy       10240  thrpt    5  40571.516 ± 548.977  ops/s
FilesCopy.copy       51200  thrpt    5  23993.334 ± 506.817  ops/s
FilesCopy.copy      102400  thrpt    5  15927.485 ± 309.081  ops/s
FilesCopy.copy      512000  thrpt    5   4207.129 ±  95.454  ops/s
FilesCopy.copy     1048568  thrpt    5   2147.046 ±  33.446  ops/s
FilesCopy.copy    10485760  thrpt    5    148.798 ±   1.329  ops/s
FilesCopy.copy   104857600  thrpt    5     14.541 ±   0.675  ops/s
FilesCopy.copy  1073741824  thrpt    5      1.270 ±   0.029  ops/s

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

Commit messages:
 - 8265175: (fs) Files.copy(Path,Path,CopyOption...) should use zero-copy on Linux

Changes: https://git.openjdk.java.net/jdk/pull/3476/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3476&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265175
  Stats: 41 lines in 1 file changed: 38 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3476.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3476/head:pull/3476

PR: https://git.openjdk.java.net/jdk/pull/3476