Allow for custom URL handlers in browse-url.

Allow for custom URL handlers in browse-url.

* lisp/net/browse-url.el (browse-url-handlers): New defcustom.
(browse-url-default-handlers): New defvar.
(browse-url): Use them.  Adapt docstring.  Issue a warning pointing to
browse-url-handlers when browse-url-browser-function is an alist.
(browse-url--mailto, browse-url--man): New functions.
(browse-url--browser-defcustom-type): Add :doc that the alist usage is
deprecated.
(browse-url-browser-function): Remove documentation referring to the
alist usage and mention browse-url-handlers.
* doc/emacs/misc.texi: Document browse-url-handlers in Browse-URL
node.
* etc/NEWS: Mention browse-url-default-handlers and
browse-url-handlers.