Link in SECURITY.html to all web pages

The location of the link is in the About menu under the 'Contacting the
IOCCC' and above 'Thanks for the help'. At first I had it at the very
bottom, under 'IOCCC markdown guidelines', but it seemed better to have
it closer to contacting the IOCCC. This is the same location in the no
JavaScript menu (which, for those who do not know or forget, is not a
requirement except for some screen sizes).

The top level Makefile has a new 'security' rule. Updated make help.

Typo fixes in make help.

SECURITY.html has had a few updates as well: change the wording of entry
code is unsupported (it now says 'ALL CODE IN ALL IOCCC ENTRIES IS
UNSUPPORTED!!') and it now links to the mkiocccentry repo and how to
report a possible security vulnerability in that repo; as that IS
supported - and since there was reference to discussing security
concerns is in SECURITY.html - even though the entries will not receive
security updates - it seemed a good idea to have a link to report such
security issues in the mkiocccentry repo. Perhaps the FAQ needs to be
updated for this too.

markdown.html

@@ -174,6 +174,12 @@
               </a>
             </div>
 
+            <div class="outfit-font">
+              <a href="./SECURITY.html" class="sub-item-link">
+                IOCCC Security Policy
+              </a>
+            </div>
+
             <div class="outfit-font">
               <a href="./thanks-for-help.html" class="sub-item-link">
                 Thanks for the help