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).

2019/dogon/README.md

@@ -225,7 +225,7 @@ character juxtaposed against a very ancient one, not to mention the program
 includes some famous name dropping and documents itself as usual.
 
 
-### Further deobfuscation, engineering, and obfuscation galore:
+### Further spoilers, engineering, and obfuscation galore:
 
 Those wait for you, curious and intrepid reader, in the attached
 [apholife.c](%%REPO_URL%%/2019/dogon/apholife.c) file, which is a partially obfuscated version of the

2019/dogon/index.html

@@ -565,7 +565,7 @@ resulting <code>F</code> in hexadecimal…</p>
 <p>And yes, the layout hint involves a mix of the most trendy typographical
 character juxtaposed against a very ancient one, not to mention the program
 includes some famous name dropping and documents itself as usual.</p>
-<h3 id="further-deobfuscation-engineering-and-obfuscation-galore">Further deobfuscation, engineering, and obfuscation galore:</h3>
+<h3 id="further-spoilers-engineering-and-obfuscation-galore">Further spoilers, engineering, and obfuscation galore:</h3>
 <p>Those wait for you, curious and intrepid reader, in the attached
 <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2019/dogon/apholife.c">apholife.c</a> file, which is a partially obfuscated version of the
 program I wrote with a lot of helpful annotations and comments for myself and