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/burton2/README.md

@@ -166,7 +166,7 @@ determine which tool was correct when there were differences.
 ```
 
 NB: `iocccsize` gets a different answer from `tac` on its own
-([iocccsize.c](%%REPO_URL%%/2018/iocccsize.c)) source code;
+[iocccsize.c](%%REPO_URL%%/2018/iocccsize.c) source code;
 `tac` gets the correct answer.  This is due to the aforementioned bugs within `iocccsize`,
 proved by fixing
 [iocccsize.c](%%REPO_URL%%/2018/iocccsize.c) with the included patch, so `iocccsize` reports
@@ -402,7 +402,7 @@ in [prog.c](%%REPO_URL%%/2018/burton/prog.c), and the program
 [manpage.sh](%%REPO_URL%%/2018/burton/manpage.sh) was added to the corpus.
 
 [manpage.sh](%%REPO_URL%%/2018/burton/manpage.sh) is a useful program that turns ASCII versions of man
-pages (e.g. [tac.man](%%REPO_URL%%/2018/burton/tac.man)) into real man pages.  It works for C and C-like
+pages (e.g., [tac.man](%%REPO_URL%%/2018/burton/tac.man) man page) into real man pages.  It works for C and C-like
 languages, command lines, and was designed to handle man page sections 1-8.  It
 works well on most inputs, but tends to need some raw nroff for more fancy
 constructions, such as combined arguments (e.g. the `-r` and `-s` arguments to tac