Let 'browse-url-interactive-arg' return more values for NEW-WINDOW-FLAG

Let 'browse-url-interactive-arg' return more values for NEW-WINDOW-FLAG

Previously it always returned t or nil for NEW-WINDOW-FLAG, but now it
can return the actual prefix arg when appropriate.  This lets functions
for 'browse-url-browser-function' consult it and do more things than
just open a new window or not (for example, you could use "C--" as the
prefix arg to do something special in a custom function).

* lisp/net/browse-url.el (browse-url-interactive-arg): Use 'xor' to
adjust the value of 'current-prefix-arg'.
(browse-url): Update docstring.