Move 2018/ferguson/prog-simple.c to prog.alt.c

This is to match the rest of the repo as this truly is an alt version,
not a supplementary program.

Updated README.md to have an 'Alternate code' section as well.

A number of files had to be changed as numerous files referred to
prog-simple.c.

2018/ferguson/.gitignore

@@ -1,5 +1,5 @@
 prog
-prog-simple
+prog.alt
 prog.orig
 weasel
-weasel-simple
+weasel.alt