save-cmd: open files with O_NOATIME on OSes that support it.

save-cmd: open files with O_NOATIME on OSes that support it.

Backing up files normally changes their atime, which is bad for two reasons.

First, the files haven't really been "accessed" in a useful sense; the fact
that we backed them up isn't an indication that, say, they're any more
frequently used than they were before.

Secondly, when reading a file updates its atime, the kernel has to enqueue
an atime update (disk write) for every file we back up.  For programs that
read the same files repeatedly, this is no big deal, since the atime just
gets flushed out occasionally (after a lot of updates).  But since bup
accesses *every* file only once, you end up with a huge atime backlog, and
this can wastefully bog down your disks during a big backup.

Of course, mounting your filesystem with noatime would work too, but not
everybody does that.  So let's help them out.