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/.gitignore

@@ -21,3 +21,5 @@ test2.args
 /ifcreate[12].log
 /ifcreate[12].dep
 /ifcreate[12]
+/broken
+/shellfile