Fix 2004/gavin/{kernel,fs.tar} commit

Landon pointed out some issues that I missed (thank you Landon .. I was
very tired and I hope that I got everything this time) in commit
4b0a2cb64a:

0.  The make clobber rule removes the git files 2004/gavin/kernel and
    2004/gavin/fs.tar.

1.  The 2004/gavin/fs.tar file contains the file "README.md" which, when
    untarred, will overwrite the git file 2004/gavin/README.md.

2.  The 2004/gavin/fs.tar file contains the file "gavin.c" which, when
    untarred, will overwrite the git file 2004/gavin/gavin.c.

3.  The 2004/gavin/fs.tar file is a tar file that could be related with
    just checking in the 3 files "prim", "sh" and "vi".

and I discovered another issue too namely that the tarball had the wrong
version of gavin.c in it as well (prior to the fix for clang).

Now the tarball is in the img/ subdirectory and the tarball contents (is
that tar ? :-) ) looks like:

    $ tar ft img/fs.tar
    fs/
    fs/gavin.c
    fs/vi
    fs/README.md
    fs/sh
    fs/prim

I added to the todo.md file that this file should be verified to match
the contents of 2004/gavin and I gave a command to do to test this - at
the end of working on this entry or maybe better yet right before this
repo is to be folded back into the winner repo.

Updated .gitignore so that the files that are in img/fs.tar are not
added to the repo. The tarball is not meant to be extracted anyway but
rather copied to a disk for usage of the entry. It will be extracted to
verify that the contents match that of the entry itself (2004/gavin
directory).

2004/gavin/.gitignore

@@ -3,3 +3,6 @@ sh
 vi
 gavin.o
 fs/
+img/README.md
+img/gavin.c
+img/prim

2004/gavin/README.md

@@ -66,7 +66,8 @@ Press and hold any key.
 
 [Yusuke Endoh](/winners.html#Yusuke_Endoh)  provided the `kernel` and `fs.tar`
 files which can be used if you cannot normally use this entry. Instead of
-generating the files just use the files provided.
+generating the files just use the files provided, found under the [img/](img/)
+directory.
 
 Thank you Yusuke!