Fix -S with -b not atomically updating the destination file.

With both of these flags, the backup was created via rename(dest, backup)
followed by rename(tmp, dest). This left the destination file missing
for a moment which contradicts the point of -S.

This fixes a race with installworld where PRECIOUSPROG and PRECIOUSLIB
files (which use -S for installation) would briefly be missing. In the
case of installing rtld with parallel installworld it could render an
error due to not having rtld present to run install/cp in another
process.

Reported by: jhb
Reviewed by: jhb
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7451