Bug 329901 Use MDC for some external links

p=Giacomo Magnini <giacomo.magnini@portalis.it> r=me

extensions/help/resources/locale/en-US/mail_sec_help.xhtml

@@ -71,7 +71,7 @@
 <p>The sections that follow provide a brief overview of how digital signatures
   and encryption work. For more technical details on this subject, see the
   online document
-  <a href="http://www.redhat.com/docs/manuals/cert-system/admin/7.1/pki.html#11284">Introduction
+  <a href="http://developer.mozilla.org/en/docs/Introduction_to_Public-Key_Cryptography">Introduction
   to Public-Key Cryptography</a>.</p>
 
 <h3 id="how_digital_signatures_work">How Digital Signatures Work</h3>

extensions/help/resources/locale/en-US/ssl_help.xhtml

@@ -176,10 +176,10 @@
 
 <ul>
   <li>
-    <a href="http://www.redhat.com/docs/manuals/cert-system/admin/7.1/pki.html#11284">Introduction
+    <a href="http://developer.mozilla.org/en/docs/Introduction_to_Public-Key_Cryptography">Introduction
     to Public-Key Cryptography</a></li>
   <li>
-    <a href="http://www.redhat.com/docs/manuals/cert-system/admin/7.1/SSL.html#11284">Introduction
+    <a href="http://developer.mozilla.org/en/docs/Introduction_to_SSL">Introduction
     to SSL</a></li>
   <li>
     <a href="http://www.mozilla.org/projects/security/pki/nss/nss-3.9/nss-3.9-algorithms.html">Encryption