Make a backup of the disk file visited by the current buffer, if appropriate.
This is normally done before saving the buffer the first time.
A backup may be done by renaming or by copying; see documentation of
variable `make-backup-files'. If it's done by renaming, then the file is
no longer accessible under its old name.
The value is non-nil after a backup was made by renaming.
It has the form (MODES SELINUXCONTEXT BACKUPNAME).
MODES is the result of `file-modes' on the original
file; this means that the caller, after saving the buffer, should change
the modes of the new file to agree with the old modes.
SELINUXCONTEXT is the result of `file-selinux-context' on the original
file; this means that the caller, after saving the buffer, should change
the SELinux context of the new file to agree with the old context.
BACKUPNAME is the backup file name, which is the old file renamed.