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.

1986/README.md

@@ -13,7 +13,7 @@ A poll was taken for the worst code for
 [BSD](https://en.wikipedia.org/wiki/Berkeley_Software_Distribution) and [System
 V](https://en.wikipedia.org/wiki/UNIX_System_V) program.  The [Bourne
 Shell](https://en.wikipedia.org/wiki/Bourne_shell) (`/bin/sh`) won for both
-systems.  The BSD [finger](https://en.wikipedia.org/wiki/Finger_(protocol))
+systems.  The BSD [finger](https://en.wikipedia.org/wiki/Finger_(protocol))
 program took third place.
 
 [Rules](rules.txt) and [results](../years.html#1986) were posted to _net.lang.c_

1986/wall/README.md

@@ -19,8 +19,8 @@ This caused a number of System V C pre-preprocessors to gripe about a
 syntax error.
 
 This program is a masterpiece of obfuscation and humor.  Larry Wall (the hacker
-who gave us [rn](https://en.wikipedia.org/wiki/Rn_(newsreader)) and
-[patch](https://en.wikipedia.org/wiki/Patch_(Unix))) asks you to compile and
+who gave us [rn](https://en.wikipedia.org/wiki/Rn_(newsreader)) and
+[patch](https://en.wikipedia.org/wiki/Patch_(Unix)), etc.) asks you to compile and
 consider the warning message generated by the C compiler.  Then execute the
 program and type characters into its standard input.  You will see why the
 compiler documents the program.

1986/wall/index.html

@@ -407,7 +407,7 @@ This caused a number of System V C pre-preprocessors to gripe about a
 syntax error.</p>
 <p>This program is a masterpiece of obfuscation and humor. Larry Wall (the hacker
 who gave us <a href="https://en.wikipedia.org/wiki/Rn_(newsreader)">rn</a> and
-<a href="https://en.wikipedia.org/wiki/Patch_(Unix)">patch</a>) asks you to compile and
+<a href="https://en.wikipedia.org/wiki/Patch_(Unix)">patch</a>, etc.) asks you to compile and
 consider the warning message generated by the C compiler. Then execute the
 program and type characters into its standard input. You will see why the
 compiler documents the program.</p>