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.