When creating a backup file, sed renamed the original before renaming the
changed copy into place, leading to a short time when no file with the
original name was present (usually only visible on SMP systems). Try
creating the backup file using a hard link instead, avoiding this problem.
If creating the hard link fails for any reason, fall back to the old rename
method.
When not creating a backup file, sed already renamed the changed copy onto
the original. This remains unchanged.
I am not adding the suppression of redundant fchown/fchmod to this commit,
because FreeBSD appears to check this in the kernel (for msdosfs at least).
PR: bin/153261
Submitted by: Pedro F. Giffuni
Reviewed by: dds (older version)
Obtained from: Illumos
MFC after: 2 weeks
919e14f01e