Rewrite to consume significantly less memory, by using find -s instead of

Rewrite to consume significantly less memory, by using find -s instead of
find | sort.  As a bonus, this simplifies the logic considerably.  Also
remove the bogus "overruning the args to ls" comment and the corresponding
"-n 20" argument to xargs; the whole point with xargs is precisely that it
knows how large the argument list can safely get.

Note that the first run of the updated script may hypotheticall produce
false positives due to differences between find's and sort's sorting
algorithm.  I haven't seen this during testing, but others might.

MFC after:	2 weeks