Because our experiment involves creating a file called redo/sh, and the
redo directory (or some other version of redo's dirctory) might be on
the path, just testing 'sh' is actually wrong: we might end up with
some earlier version of redo-sh.
Instead, specifically for sh, always demand that we test /bin/sh.
I feel a little dirty doing this, but the way the code was before, redo
almost always picked bash as the shell. bash is way too overpowered
and this led to bashisms in do scripts unnecessarily. The two failures
in dash are things that I would really like to have, but they haven't
materialized after 6 years, so I guess we should be realistic.
To appropriately penalize bash for asking for trouble, I added a
warning about [ 1 == 1 ] syntax being valid (as opposed to the POSIX
correct [ 1 = 1 ]). This allows dash to be selected ahead of bash.
I also moved 'sh' to the end of the list, because although it's the
weakest shell on some systems, on other systems it's just bash. And I
put zsh in front of bash, because fewer people have zsh and we want
them to test zsh.
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.
