Since the changes of 2004-05-22, the u.saved_cwd member at

Since the changes of 2004-05-22, the u.saved_cwd member at
the bottom of the active-directory stack was no longer
strictly necessary.  This change removes that member and uses
the newer cwd_state parameter for the final restore_cwd.

(struct AD_ent) [u]: Remove now-unnecessary union.
[dev_ino]: Rename from `a'.
(AD_pop_and_chdir): Add a parameter to play the role of just-removed
bottom-of-stack cwd-state member.
(AD_pop_and_chdir): No longer return boolean.  Adjust caller.
(AD_push_initial): Remove CWD parameter.  Adjust caller.