updatecheck feature

This patch adds a feature to let DokuWiki automatically check if updates are
available or any other important messages (like security warnings) and then
display this info to the admin user.

DokuWiki will contact the URL http://www.splitbrain.org/lib/exe/msg.php
with a parameter telling it which messages it already know (read from
conf/msg) - the server side script then will return all new messages.

The messages will be displayed until DokuWiki was upgraded or conf/msg
was updated manually. Messages are cached and only checked once a day.

The messenger URL will probably change before the next release.

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