* lisp/thingatpt.el (thing-at-point): Accept optional second argument

* lisp/thingatpt.el (thing-at-point): Accept optional second argument
NO-PROPERTIES to strip the text properties from the return value.
* lisp/net/browse-url.el (browse-url-url-at-point): Pass NO-PROPERTIES
to `thing-at-point' instead of stripping the properties ourselves.
Also, when `thing-at-point' fails to find a url, prepend "http://"
to the filename at point on the assumption that the user is
pointing at something like gnu.org/gnu.