RSS patch for Thunderbird (and some RSS clients).

In the RSS specifications, the "link" tags contains the URL to follow,
and the "guid" contains a unique identifier (which may or may not be an
URL).
RSS clients should always use "link" to follow the link (and most do),
but Thunderbird uses the "guid" if it find a valid URL inside (and only
falls back to "link" if "guid" is not an URL).

I have patched the RSS feed so that Thunderbird ignores the URL in guid.