manually touch atime when reading a mbox file

The only common use of atime left is local mail agents leaving a mark on
mbox file after the mail has been read.  And, since POSIX-2008, it is
possible to use futimens() to alter atime even on filesystems mounted with
noatime.

There's no extra cost for doing this on when atime updates are enabled:
the inode will be dirty already.