Automatically select a good shell instead of relying on /bin/sh.

This includes a fairly detailed test of various known shell bugs from the
autoconf docs.

The idea here is that if redo works on your system, you should be able to
rely on a *good* shell to run your .do files; you shouldn't have to work
around zillions of bugs like autoconf does.

test.do

@@ -1,2 +1,2 @@
-redo-ifchange Documentation/all
+redo-ifchange _all
 redo t/test