* helm-dabbrev.el: Implement cycling in first candidates before helm completion.
* helm-dabbrev.el (helm-dabbrev-cycle-thresold): New user var, disable cycling when 0 or null.
* helm-dabbrev.el: Remove hardcoded value for helm-dabbrev-cycle-thresold.
* helm-dabbrev.el (helm-dabbrev-cycle-thresold): Fix docstring.