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.

1990/baruch/README.md

@@ -40,13 +40,13 @@ Turbo-C or MSC, the code is based on the authors' remarks except that the `"
 improved output though admittedly we have no way to test the compilers in
 question. YMMV.
 
-#### To build:
+### Alternate build:
 
 ```sh
 make alt
 ```
 
-#### To use:
+### Alternate use:
 
 Use `baruch.alt` as you would `baruch`.