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/Makefile

@@ -15,7 +15,8 @@ PREFIX ?= /usr/local
 SHELL=/bin/bash
 
 MAIN=cameo
-OBJS=cameo.o excellon.o gerber.o gnuplot.o ops.o path.o lex.yy.o y.tab.o
+OBJS=cameo.o excellon.o gerber.o gnuplot.o ops.o path.o shape.o \
+     lex.yy.o y.tab.o
 
 CFLAGS_WARN=-Wall -Wshadow -Wmissing-prototypes \
             -Wmissing-declarations -Wno-format-zero-length