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.

t/shelltest.do

@@ -0,0 +1,8 @@
+set +e
+( . ./shelltest.od )
+RV=$?
+case $RV in
+	0) exit 0 ;;
+	42) exit 0 ;;
+	*) exit 1 ;;
+esac