Categorize browse-url functions into internal and external ones.

Categorize browse-url functions into internal and external ones.

* lisp/net/browse-url.el: Write package documentation explaining
browse-url-browser-kind symbol property.  Categorize existing
browse-url functions into internal and external ones.
(browse-url--browser-kind, browse-url--browser-kind-mailto)
(browse-url--browser-kind-man, browse-url--browser-kind-browser): New
functions.
(browse-url-select-handler): Add KIND argument to restrict selection.
* lisp/dnd.el (dnd-handle-one-url): Only select browse-url handler of
kind `internal'.
* lisp/net/eww.el (eww): Add `browse-url-browser-kind' symbol property
with value `internal'.