minimal/do: make redo-ifchange (etc) into subprograms in a temp dir.

Using aliases for them was cute, but it didn't work with things like:

	find -name '*.c' | xargs redo-ifchange

since xargs doesn't know about aliases.

clean.do

@@ -3,6 +3,7 @@ if [ -e .do_built ]; then
 		rm -f "$x"
 	done <.do_built
 fi
+[ -z "$DO_BUILT" ] && rm -rf .do_built .do_built.dir
 redo t/clean
 rm -f *~ .*~ */*~ */.*~ *.pyc
 rm -rf t/.redo