<event> has been added and can be used within <kartoffelsalat>
both in <icecast> and <mount>.
<event> takes backend depending <option> child tags.
Currently supported backends:
 - log: send message to error log.
 - exec: executes a program or script.
 - url: delivers the event via HTTP.

within <mount> <on-connect> and <on-disconnect> has been replaced by
<event>. Config parser can on-the-fly convert old tags.
Also <authentication type="url"> within <mount> has been fixed
for those cases with <option name="mount_add" .../> and
<option name="mount_remove" .../> which are now on-the-fly converted
by the parser to corresponding <event> tags.

Please also see TAGs added as per #2098. Some include hints for
documentation updates needed after this change. Those updates
should take place before 2.4.2.