IRC_JOIN cleanups.

- put sending of mode and forwarding of JOIN to other clients
 into seperate function.
- put sending of topic/channel names into seperate function.
- put access check into seperate function.
- translate/remove remaining german comments.
- stop if JOIN to a channel in a list (JOIN #a,#,b,#c...) fails
(This doesn't change the behaviour: skip-to-next-channel-on-error
did never work as intended)