cmd_TERM: also kill AsyncIO contexts
- the next_pid var has to be global, so we have a uniq source of session numbers.
- during cloning sessions, cached_msglist and CIT_ICAL mustn't be copied.
- FreeAsyncIOContents(): NULL pointer to context after flagging it for deletion.
- InitIOStruct() / InitcURLIOStruct(): IO and CC are now doublepointered, know each others.
- KillAsyncIOContext () detect in which event-queue the context is running
- Join that queue, and call the terminator hook of the other session.
- self-terminate afterwards.