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.

builder.py

@@ -197,7 +197,8 @@ class BuildJob:
         after_t = _try_stat(t)
         st1 = os.fstat(f.fileno())
         st2 = _try_stat(self.tmpname2)
-        if after_t != before_t and not stat.S_ISDIR(after_t.st_mode):
+        if (after_t and after_t != before_t and 
+            not stat.S_ISDIR(after_t.st_mode)):
             err('%s modified %s directly!\n' % (self.argv[2], t))
             err('...you should update $3 (a temp file) or stdout, not $1.\n')
             rv = 206