fix many internal links

Fixed a number of internal links that need to
go to the GitHub site.

Ran `make www` to perform the above.

1986/marshall/README.md

@@ -27,7 +27,7 @@ if nothing else.
 
 Due to the different [conflicting problems](compilers.html) with gcc and clang, we
 instead offer the problematic code as an alternate version whereas
-[marshall.c](marshall.c) has both the infinite loop and the complicated arg to
+[marshall.c](%%REPO_URL%%/1986/marshall/marshall.c) has both the infinite loop and the complicated arg to
 `_exit()` commented out, changing the value passed into `_exit()` to `1`.