try to fix #593

Seems to work ok for me; two problems here, though:

  - if you add ~user to the  minibuffer (without final slash) and hit RET, it doesn't work as intended.  Instead, helm creates a file names "~user" in your current dir

  - Indenpendent from this, but among these lines: you added this line to fix #424:
````
    (skip-chars-forward "//") ;; Avoid infloop in UNC paths Issue #424
````
  This is equivalent to
````
    (skip-chars-forward "/")
````
  (one slash), no?  Is there something different intended?