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/clean.do

@@ -1,6 +1,6 @@
 redo example/clean curse/clean deps/clean "space dir/clean" stamp/clean \
 	defaults-flat/clean
-rm -f mode1 makedir.log chdir1 deltest2 \
+rm -f broken shellfile mode1 makedir.log chdir1 deltest2 \
 	hello [by]ellow *.o *~ .*~ *.log CC LD passfail silence silence.do \
 	touch1 touch1.do always1 ifcreate[12].dep ifcreate[12]
 rm -rf makedir