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.

2018/poikola/index.html

@@ -440,8 +440,8 @@ better with it.</p>
 best viewing experience is achieved when the terminal in use supports 24-bit
 colors, has a black background and the size is at least 125x38.</p>
 <p>Special note for Mac users: <code>Terminal.app</code> does not work as expected; you might
-need xterm from XQuartz (see <a href="../../faq.html#X11macos">FAQ 3.7: How do I compile and use on macOS, an
-IOCCC entry that requires X11?</a>) or some other working
+need xterm from XQuartz, see <a href="../../faq.html#X11macos">FAQ 3.7: How do I compile and use on macOS, an
+IOCCC entry that requires X11?</a>, or some other working
 terminal. Thanks to <a href="../../authors.html#Dave_Burton">Dave Burton</a> for spotting this
 problem.</p>
 <p>The main reason for the header <code>unistd.h</code> is <code>getdelim(3)</code> but once I included it