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.
So that more than one redo doesn't try to build the same thing at the same
time. Kind of dumb, though, since it currently wipes out all the locks at
the toplevel, so running more than one at a time won't give accurate
results, but the -j option doesn't do anything yet.
