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.

1995/garry/README.md

@@ -35,14 +35,14 @@ judges noted that the C pre-processed version (with the `#include`s
 left intact) looked very much like a rat "dropping core".  See
 [garry.alt.c](garry.alt.c) and judge for yourself!
 
-#### To build:
+### Alternate build:
 
 
 ```sh
 make alt
 ```
 
-#### To use:
+### Alternate use:
 
 ```sh
 ./garry.alt <input_file >output_file