Invent a cunning means of faking plausible atimes for directories,

since directories will tend to have been accessed constantly by
other recursive disk scans and hence not really reflect the semantic
last-usage status of the stuff within them. Directories' atimes are
now computed as the maximum of their mtime and all the atimes below
them. (There's a command-line option to revert to the obvious
behaviour, but I think it is not in general the most useful thing.)

git-svn-id: svn://svn.tartarus.org/sgt/agedu@8255 cda61777-01e9-0310-a592-d414129be87e