Bind `helm--reading-passwd-or-string' when reading-string during helm session (#544).
* helm.el (helm-read-string): New.
(helm--reading-passwd-or-string): Renamed from `helm--reading-passwd'.
* helm-bbdb.el:     Use it.
* helm-external.el: Use it.
* helm-files.el:    Use it.
* helm-grep.el:     Use it.
* helm-regexp.el:   Use it.
* helm-w3m.el:      Use it.