Change alt/orig to build/to use heading levels

As the 'Alternate code' and 'Original code' heading level is now 2 the
'To build' and 'To use' subsections should be level 3, not 4, and also
reworded, this was done:

    sgit -e 's/^#### To build/### Alternate build/g' -e 's/^#### To use/### Alternate use/g' '*README.md'

Many more have to be updated and this will be done manually at another
time.

1986/marshall/README.md

@@ -98,7 +98,7 @@ If the optimiser is enabled, however, we will see that it works fine before the
 change to `_exit()`.
 
 
-#### To use:
+### Alternate use:
 
 Due to the different conflicting problems with gcc and clang, we instead offer
 the problematic code as an alternate version whereas [marshall.c](marshall.c)