user-util: add generic definition for special password hash values in /etc/passwd + /etc/shadow

Let's add three defines for the 3 special cases of passwords.

Some of our tools used different values for the "locked"/"invalid" case,
let's settle on using "!*" which means the password is both locked *and*
invalid.

Other tools like to use "!!" for this case, which however is less than
ideal I think, since the this could also be a considered an entry with
an empty password, that can be enabled again by unlocking it twice.