Revert "use /event/-based singleton mutex to reload wmiirc instead of…

… exec()"

Because if zombie instances of this wmiirc have an IXP::Error on
/event, then there's no way they are going to see "Start wmiirc"
on /event and kindly terminate themselves.

This reverts commit a71b748.

config.rb

@@ -439,5 +439,5 @@ def status_click name, mouse_button
 #
 def reload_config
   LOG.info 'reload'
-  launch $0
+  exec $0
 end