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).

1998/df/README.md

@@ -111,20 +111,22 @@ I decided it would be better this way...
 These restrictions have been removed for your convenience.
 
 The following lines describe how this program works; if you don't
-want to know it at this time, please skip them.
+want to know it at this time, please skip them. If this entry would
+by some mistake win the contest, please do not rot13 this - I hate
+rot13 spoilers since it is somewhat complicated on a Mac to
+reverse them. Anyone out there should have enough discipline and
+rot13 shouldn't be required, really. After all, we are not children
+anymore :-)
 
-You should have enough discipline to make an effort to understand
-the code without reading the deobfuscation information below.
-After all, we are not children anymore :-)
 
-## Deobfuscation information
+## Obfuscation information:
 
 <hr style="width:10%;text-align:left;margin-left:0">
 
 This paragraph is not part of the author's remarks, but is a helpful
 gift from the judges; if you read too fast to stop yourself, this should
 give you enough time to stop, which you really need to do RIGHT NOW if
-you don't want to read the deobfuscation information: we mean it.
+you don't want to read the obfuscation information: we mean it.
 
 <hr style="width:10%;text-align:left;margin-left:0">