* src/login.c: Get rid of pwent. pwd is sufficient as long as it

	* src/login.c: Get rid of pwent. pwd is sufficient as long as it
	is always coming from xgetpwnam. There is no need to copy pwd to
	pwent, this was not a good idea anyway as the strings from pwd
	were not duplicated.
	* src/login.c: Always free the pwd and spwd structure when we
	retrieve a new one. This will clear the password of the previous
	user from the memory.
	* src/login.c: user_passwd is used to keep point to the password
	of the user being authenticated.
	* src/login.c: (non PAM) Fail if the user's entry cannot be found
	after the user updated her password (if expire() requested an
	update).
	* src/login.c: If the user does not exist on the system, there is
	no need to build a pwd structure (with shell).