sysusers: use "!*" instead of "!!" as an invalid group password

This basically implements fc58c0c7bf for gshadow.
gpasswd may not have a lock/unlock that behaves the same as passwd, but
according to gshadow(5) the logic of the password field is the same.

src/sysusers/sysusers.c

@@ -693,7 +693,7 @@ 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*) "!*",
                 };
 
                 r = putsgent_with_members(&n, gshadow);