update.pl is given the name of a collection that is based on a
web site and updates the local mirror of that web site if appropriate.
It looks in the etc/collect.cfg file for a line beginning with
"mirror interval " followed by the number of days between mirrors.
If a suitable number of days have passed then it calls mirror.pl
for the collection to update the mirror, then uses import.pl and
buildcol.pl to bring the indexes up to date.