add hack-a-round for language specified markdown code blocks

When language is specified in a markdown code block such as:

````
```c
    printf("Hello, world!\n");
```
````

is used, the resulting HTML as produced by `pandoc(1)` combined
with our revised `ioccc.css` is not intent the code block well.

A temporary "_hack-a-round_" is to turn the "markdown code language
specifier" into an HTML comment.  For example:

````
```<!---c-->
    printf("Hello, world!\n");
```
````

This commits turns off existing markdown code language specifiers
for now, while preserving our ability to restore them in the future
when a better  "_hack-a-round_" to the "markdown code language
specifier" problem is found.

The HTML impact if this "_hack-a-round_" is to produce HTML that
looks like this:

```
<pre class="&lt;!---c--&gt;"><code> .. </code></pre>
```

We might be able to create a complex set of classes for each markdown
code language (there won't be very many) in `ioccc.css`, or we might
find a more general CSS (or maybe `pandoc(1)`?) solution.

For now, this hack-a-round will have to do.

1998/schweikh1/macos.md

@@ -19,7 +19,7 @@ However, as noted, macOS does **not** have `/usr/include` so this would never
 work for macOS without some changes, described next. For macOS you will need the
 command line tools installed which can be done like:
 
-```sh
+```<!---sh-->
     sudo xcode-select --install
 ```
 
@@ -50,7 +50,7 @@ which has been changed to be:
 
 Shortly below that you will see, as noted above, the numbers:
 
-```c
+```<!---c-->
     if ((H = fopen (__FILE__+43, 43+__FILE__)))
     while ((fgets (L, (int)sizeof L, H)) != 0) {
 	    I[strcspn (I, 45+__FILE__)] = O = 0;