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/sysusers/sysusers.c

@@ -441,7 +441,7 @@ static int write_temporary_passwd(const char *passwd_path, FILE **tmpfile, char
                         .pw_gecos = i->description,
 
                         /* "x" means the password is stored in the shadow file */
-                        .pw_passwd = (char*) "x",
+                        .pw_passwd = (char*) PASSWORD_SEE_SHADOW,
 
                         /* We default to the root directory as home */
                         .pw_dir = i->home ?: (char*) "/",
@@ -551,7 +551,7 @@ static int write_temporary_shadow(const char *shadow_path, FILE **tmpfile, char
 
                 struct spwd n = {
                         .sp_namp = i->name,
-                        .sp_pwdp = (char*) "!*", /* lock this password, and make it invalid */
+                        .sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID,
                         .sp_lstchg = lstchg,
                         .sp_min = -1,
                         .sp_max = -1,
@@ -682,7 +682,7 @@ static int write_temporary_group(const char *group_path, FILE **tmpfile, char **
                 struct group n = {
                         .gr_name = i->name,
                         .gr_gid = i->gid,
-                        .gr_passwd = (char*) "x",
+                        .gr_passwd = (char*) PASSWORD_SEE_SHADOW,
                 };
 
                 r = putgrent_with_members(&n, group);
@@ -766,7 +766,7 @@ static int write_temporary_gshadow(const char * gshadow_path, FILE **tmpfile, ch
         ORDERED_HASHMAP_FOREACH(i, todo_gids) {
                 struct sgrp n = {
                         .sg_namp = i->name,
-                        .sg_passwd = (char*) "!*",
+                        .sg_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
                 };
 
                 r = putsgent_with_members(&n, gshadow);