Change definitions of $1,$2,$3 to match djb's redo.

If you use "redo --old-args", it will switch back to the old
(apenwarr-style) arguments for now, to give you time to update your .do
scripts.  This option will go away eventually.

Note: minimal/do doesn't understand the --old-args option.  If you're using
minimal/do in your project, keep using the old one until you update your use
of $1/$2, and then update to the new one.

apenwarr-style default.o.do:
   $1      foo
   $2      .o
   $3      whatever.tmp

djb-style default.o.do:
   $1      foo.o
   $2      foo
   $3      whatever.tmp

apenwarr-style foo.o.do:
   $1      foo.o
   $2      ""
   $3      whatever.tmp

djb-style foo.o.do:
   $1      foo.o
   $2      foo.o  (I think?)
   $3      whatever.tmp

redo.py

@@ -15,6 +15,7 @@ shuffle    randomize the build order to find dependency bugs
 debug-locks  print messages about file locking (useful for debugging)
 debug-pids   print process ids as part of log messages (useful for debugging)
 version    print the current version and exit
+old-args   use old-style definitions of $1,$2,$3 (deprecated)
 """
 o = options.Options(optspec)
 (opt, flags, extra) = o.parse(sys.argv[1:])
@@ -39,6 +40,8 @@ if opt.debug_locks:
     os.environ['REDO_DEBUG_LOCKS'] = '1'
 if opt.debug_pids:
     os.environ['REDO_DEBUG_PIDS'] = '1'
+if opt.old_args:
+    os.environ['REDO_OLD_ARGS'] = '1'
 
 import vars_init
 vars_init.init(targets)