Description of problem:
Setting up in /etc/shadow :
 --
foo:pass:13454:0:99999:7::0:
 --

Will make chage not show the Account Expires field, as follows :
 --
Account Expires:        Never 
 --

My understanding is that Linux' based tools use -1 as the undefined value,
however on other UNIX platforms, 0 was used.
Checking on the source code there is still a bad condition that got fixed
upstream and is also fixed on RHEL4.

 --
if (expdays <= 0) {
 --

Which should be < to allow just -1. The fact 0 is included on the test makes
that date not shown, even if being that date a wrong setup, as by logic you
would expect the system administrator to have date + Days setup.
Otherwise the account gets blocked.

However, if there is a legacy application ported from another UNIX, that uses
shadow somehow and sets 0, then the account reset will be refused to access.

Version-Release number of selected component (if applicable):
shadow-utils-4.0.3-26.RHEL3

How reproducible:
Always

Steps to Reproduce:
1. Add a test user.
2. Edit manually /etc/shadow adding 0 in the sp_expire field.
3. chage the user. chage -l foo. 
  
Actual results:
Account Expires: Never.

Expected results:
Account Expires: Jan 01, 1970.

Additional info:
Patch attached.