* Change the semantics of the `-s' option somewhat. If specified, allow
either Kerberos or S/Key login, but not a plain password.
* Eliminate the special `s/key' password; just type it at the prompt.
* Remove the root instance special case. This is a serious security hole
waiting to happen, and no other system works this way.
* Don't force a password change if Kerberos was used. Also, don't call
/bin/passwd at all if the password change isn't required.
* if the user has an s/key, provide a reminder in the password prompt
* if '-s' is given once, force a user that has an s/key to use it
* if '-s' is given more than once, only permit s/key logins
