user-util: return malloc'ed strings in get_user_creds

get_user_creds would use getpwnam() and then returns strings pointing
into the static buffer. This seems very iffy. Let's duplicate the
strings properly

src/shared/condition.c

@@ -396,8 +396,7 @@ static int condition_test_user(Condition *c, char **env) {
         if (streq(username, c->parameter))
                 return 1;
 
-        const char *u = c->parameter;
-        r = get_user_creds(&u, &id, NULL, NULL, NULL, USER_CREDS_ALLOW_MISSING);
+        r = get_user_creds(c->parameter, USER_CREDS_ALLOW_MISSING, NULL, &id, NULL, NULL, NULL);
         if (r < 0)
                 return 0;