Reword the recent deobfuscation changes

In most cases the word was changed to 'obfuscation' as it was about
obfuscation, not deobfuscation. In some cases like 1995/leo since the
file was (in 1995/leo) about a secret switch it was changed to
'secret.md' (and 'secret.html') but there were others like that as well.

One entry had the words 'spoiling the size rule' but this had been
changed to 'deobfuscating the size rule' which was obviously wrong.

In the latter YYYY/README.md files the judges had written after some
entries (brief summaries) '...

    ...We'll stop spouting spoilers now. Have fun exploring all the
    entries!

but that was changed to:

    ...We'll stop spouting deobfuscation information now. Have fun
    exploring all the entries!

which wasn't really about deobfuscation but just a brief summary of what
the entries did.

And in the most problematic case (at least for me :-), 2018/ferguson) I
had put a joke in:

    ... but it's the only Easter egg I won't spoil

but it was changed to 'won't deobfuscate' which not only is not what it
is but it spoilt the joke.

In some cases the word was changed back to 'SPOILER' (or similar) but in
most cases not. I believe it should be better now. There are some
caveats, however, as below.

Although make find_missing_links is okay I do not remember if this will
catch links that refer to '%%REPO_URL%%' or other pandoc tokens. This
means I might have to go back to some earlier years to verify links are
okay. That should not take that much time I hope.

It might also be that some wording can still be improved some but it is
hoped this will be discovered when checking links and looking over the
manifest (which obviously changed).

1993/plummer/index.html

@@ -481,7 +481,7 @@ return causes the line just written to be overwritten. <code>lint</code> (and mo
 compilers) complain that there is no control path leading to a return from
 <code>main()</code>. At 125 characters, it still fits on a single line (for those of us
 who don’t mind ruining our eyes with 132 column screens).</p>
-<h3 id="deobfuscation">Deobfuscation:</h3>
+<h3 id="obfuscation">Obfuscation:</h3>
 <p>The first statement points the base of <code>l</code> (the argument vector) to
 be the first argument by incrementing it. It then assigns <code>O</code> to
 point to the character before the first character of the second