Allow dot in cron.update to re-scan whole directory

[gagern]

It's kind of tedious if one has to explicitely name every changed file, particularly in case of package installs or uninstalls. And having uninstalled packages emit error messages for up to an hour is suboptimal as well. It would be better if package managers could help us here, by triggering a re-scan of the whole directory afer installs or uninstalls.

The documentation also failed to mention /etc/cron.d/cron.update before, causing me to assume that /var/spool/cron/crontabs/cron.update could somehow be used to trigger a re-scan of /etc/cron.d as well. It should be clearer now.

Code is untested, but the change looks simple enough.