This is a departure from how djb seems to have it set up, but I just like it
better. It's more like the reasonably-common Makefile standard. (Although
what make *actually* does is just use the first target declared in the
file.)
And move test scripts into t/ subdir to keep things clean.
As part of that, make sure redo adds itself to the PATH on startup so that
subscripts can find it.
