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.

src/basic/user-util.h

@@ -111,3 +111,12 @@ int putsgent_sane(const struct sgrp *sg, FILE *stream);
 bool is_nologin_shell(const char *shell);
 
 int is_this_me(const char *username);
+
+/* A locked *and* invalid password for "struct spwd"'s .sp_pwdp and "struct passwd"'s .pw_passwd field */
+#define PASSWORD_LOCKED_AND_INVALID "!*"
+
+/* A password indicating "look in shadow file, please!" for "struct passwd"'s .pw_passwd */
+#define PASSWORD_SEE_SHADOW "x"
+
+/* A password indicating "hey, no password required for login" */
+#define PASSWORD_NONE ""