Disable by default fuzzy matching in symbol completion.

* helm-elisp.el (helm-lisp-fuzzy-completion): New user var.
(helm-lisp-completion-at-point): Use it.

* helm-command.el: Disable fuzzy in M-x.
Allow fuzzy matching in ido virtual buffers.

* helm-buffers.el (helm-source-ido-virtual-buffers): Define it with helm-source,
Allow fuzzy matching.