mtime's nsec are truncated on OpenBSD and FreeBSD (at least)

[moviuro]

Hello,

#891 and #992 did some changes to the time-copy functions, but OpenBSD and FreeBSD (at a minimum) look like they were left out.

### OpenBSD, on FFS
# gzip -fk -9 obsd.html
# brotli -kf9 obsd.html
# zstd -fkT0 -19 obsd.html
obsd.html            : 52.08%   (   891 B =>    464 B, obsd.html.zst)          
# stat -f "%Fm %z %N" obsd.html*
1785531532.061542134 891 obsd.html
1785531532.000000000 387 obsd.html.br
1785531532.061542134 461 obsd.html.gz
1785531532.061542134 464 obsd.html.zst
# brotli -V
brotli 1.2.0

cf. https://github.com/openbsd/ports/tree/master/archivers/brotli

### FreeBSD 14.4 and 15.0, on ZFS
# man 1 brotli > brotlitest
# brotli brotlitest
# zstd -fkT0 brotlitest
brotlitest           : 40.65%   (  5.60 KiB =>   2.28 KiB, brotlitest.zst)
# stat -f '%Fm %N' brotlitest*     
1785531998.940700000 brotlitest
1785531998.000000000 brotlitest.br
1785531998.000000000 brotlitest.zst
# brotli -V
brotli 1.2.0

cf. https://github.com/freebsd/freebsd-ports/tree/main/archivers/brotli

• Affected: OpenBSD, FreeBSD, maybe more? I don't have any other machines to test this on!
• Unaffected: Linux (Archlinux)