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.

2004/gavin/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
@@ -25,7 +25,7 @@ For more detailed information see [2004 gavin bugs](../../bugs.html#2004_gavin).
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./gavin
 ```
 
@@ -36,14 +36,14 @@ To enjoy the results (on a Linux/x86 machine) with a floppy drive (remember
 those? We do!):
 
 
-```sh
+```<!---sh-->
     su
 ```
 
 Make a floppy with an ext2 filesystem (`mke2fs /dev/fd0` replacing `fd0` with
 whatever the floppy device is!),  mount it under `/mnt/floppy` and then:
 
-```sh
+```<!---sh-->
     cp kernel fs.tar lilo.conf boot.b /mnt/floppy
 ```
 
@@ -51,26 +51,26 @@ NOTE: If the version of your lilo is not 21.4, use the appropriate boot.b file.
 
 NOTE: If your floppy drive is not `/dev/fd0`, edit lilo.conf appropriately.
 
-```sh
+```<!---sh-->
     lilo -C /mnt/floppy/lilo.conf
 ```
 
 Boot from the floppy on an x86 machine with a PS/2 keyboard and mouse.
 Move the window away from the corner.
 
-```sh
+```<!---sh-->
     vi gavin.c
 ```
 
 Scroll up and down the file with up-arrow and down-arrow.
 
-```sh
+```<!---sh-->
     sh
 ```
 
 You can have several shell windows:
 
-```sh
+```<!---sh-->
     vi README.md
     ./prim
 ```
@@ -86,7 +86,7 @@ you can use it with `QEMU`.
 
 ### Alternate build:
 
-```sh
+```<!---sh-->
     make alt
 ```