open files with O_NOATIME if possible

The Linux manpage for `open(2)` describes `O_NOATIME` thus:

       O_NOATIME (since Linux 2.6.8)
              Do  not  update the file last access time (st_atime in the inode) when the file is read(2).  This flag
              is intended for use by indexing or backup programs, where its use can significantly reduce the  amount
              of  disk  activity.  This flag may not be effective on all filesystems.  One example is NFS, where the
              server maintains the access time.

Change attic to provide this flag where possible.

Unrelatedly, fix a typo in a comment.