fix more broken markdown links

We fix more markdown links in markdown files that refer to files
internally to the web site.

Added tmp tools to find and print various markdown links:

* `tmp/md-links.sh` - print all markdown links
* `tmp/md-token-links.sh` - print all markdown links that start with a **%%TOKEN%%**
* `tmp/md-local-links.sh` - print all markdown links to a local file
* `tmp/md-link-paths.sh` - print local file paths referenced in markdown links

Fixed a number of markdown formatting problems with `thanks-for-help.md`.
Fixed markdown links to files in `thanks-for-help.md`.  Improved
heading levels in `thanks-for-help.md`, and made entry sections consistent.

Changed all markdown links that end in "))" to end in a better way.

Explained in `markdown.md` why we are avoiding markdown links that
end in "))".

Ran `make www` to perform the above.

TODO: Fix stuff we broke by mistake doing the above.

TODO: Fix more broken markdown links.

2019/duble/README.md

@@ -239,8 +239,8 @@ function was enough to fix this compilation issue.
 However, that was not enough to make the program work on macOS. If you analyse
 the program you will see that it heavily relies on OS resources. And, for this
 first version of the program, you even needed to increase default `sysctl`
-parameters to make it work on FreeBSD (this was the purpose of file
-[check-os.sh](%%REPO_URL%%/2019/duble/check-os.sh)). I tried hard to tune macOS the same, but failed.
+parameters to make it work on FreeBSD (this was the purpose of the
+[check-os.sh](%%REPO_URL%%/2019/duble/check-os.sh) file). I tried hard to tune macOS the same, but failed.
 
 The simple fact the Judges proposed this update meant they wish it could work on
 Mac.  And if the judges wish something, it **has to** be done. ;) So I

2019/duble/index.html

@@ -538,8 +538,8 @@ function was enough to fix this compilation issue.</p>
 <p>However, that was not enough to make the program work on macOS. If you analyse
 the program you will see that it heavily relies on OS resources. And, for this
 first version of the program, you even needed to increase default <code>sysctl</code>
-parameters to make it work on FreeBSD (this was the purpose of file
-<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2019/duble/check-os.sh">check-os.sh</a>). I tried hard to tune macOS the same, but failed.</p>
+parameters to make it work on FreeBSD (this was the purpose of the
+<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2019/duble/check-os.sh">check-os.sh</a> file). I tried hard to tune macOS the same, but failed.</p>
 <p>The simple fact the Judges proposed this update meant they wish it could work on
 Mac. And if the judges wish something, it <strong>has to</strong> be done. ;) So I
 refactored a little more the program to reduce OS resources consumption. Or