Pull up following revision(s) (requested by tonnerre in ticket #817):
lib/libpam/modules/pam_unix/pam_unix.c: revision 1.13
Restore the good old UNIX behavior of root password changing: only root
may change the root password.
(Checked that everybody else's password can be changed without any problem,
and checked that root can still change the root password.)

(bouyer)