cameo: adding support for dril/mill substitutions (in progress)

- Makefile (OBJS): added shape.o
- README: added section for drill/mill substitutions
- lang.l, lang.y: added commands "drill", "empty", "mill", "remainder", and
  "reset"
- lang.y (clear_paths): moved freeing of all paths from "clean" to shared
  function clear_paths
- ops.h, ops.c (try_drill, try_mill): helper functions for "drill" and
  "mill" commands
- shape.h. shape.c (slot, circle): functions to generate toolpaths for
  basic shapes

cameo/shape.h

@@ -0,0 +1,25 @@
+/*
+ * shape.h - Toolpaths for basic shapes
+ *
+ * Written 2010 by Werner Almesberger
+ * Copyright 2010 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#ifndef SHAPE_H
+#define SHAPE_H
+
+#include "path.h"
+
+
+struct path *slot(double xa, double ya, double xb, double yb, double z,
+    double sr, double tr, double step);
+struct path *circle(double cx, double cy, double cz, double cr, double tr,
+    double step);
+
+#endif /* !SHAPE_H */