use mm-url.el functions for external URL

use mm-url.el functions for external URL
loading when the built-in HTTP GET is insufficient (e.g. proxies
are in the way).  From Eric Knauel
<knauel@informatik.uni-tuebingen.de>.
(spam-report-url-ping-function): new option, defaults to the
built-in HTTP GET (spam-report-url-ping-plain)
(spam-report-url-ping): calls spam-report-url-ping-function now
(spam-report-url-ping-plain): new function, does what
spam-report-url-ping used to do
(spam-report-url-ping-mm-url): function that delegates to
mm-url.el (autoloaded)