internal,os: employ copy_file_range(2) for file-to-file copying on FreeBSD
FreeBSD 13.0 introduced the Linux-compatible copy_file_range(2) system call,
we should make use of it.
Ref:
https://www.gnu.org/software/gnulib/manual/html_node/copy_005ffile_005frange.html
https://reviews.freebsd.org/D20584?id=60021
https://man.freebsd.org/cgi/man.cgi?copy_file_range(2)
Change-Id: I75edb5629717289c8887be436613d3a8b3820bdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/604655
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>