Rewrite using save-cwd.c and chdir to remove quadratic component of complexity.

Rewrite using save-cwd.c and chdir to remove quadratic component of complexity.
Before, it processed O(n^2) directory name components via stat and mkdir.
Now it's O(n).  This makes mkdir -p a lot more efficient when creating
directories with very many components.  On a Linux 2.0.30 ext2fs filesystem
this command: mkdir -p `perl -e 'print "a/" x 500'`  now runs in 0.77 seconds
(user+sys).  Contrast that with the 9.5(!) seconds it took before.