Cleanup: Moved config re-read function into right place

Renamed event_config_read() into config_reread_config() and moved
it into cfgfile.c. This allowed to delete event.[ch].
event.[ch] will later be used to implement <event>.
Added <event>: Unified handling of events.

<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 has been replaced by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.
Speedup: Don't copy data into event if there are no registrations

If there are no reistrations that are added to an event the event
doesn't need to be emited as nobody cares anyway. We will do this
simple check before we copy data into the event object to avoid
unnecessary calls to strdup() and other allocation functions.