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.

clean.do

@@ -6,5 +6,5 @@ fi
 [ -z "$DO_BUILT" ] && rm -rf .do_built .do_built.dir
 redo t/clean Documentation/clean
 rm -f *~ .*~ */*~ */.*~ *.pyc install.wrapper
-rm -rf t/.redo
+rm -rf t/.redo redo-sh
 find . -name '*.tmp' -exec rm -fv {} \;