Clear passwords provided via arguments

If a password is gived as an argument, that password is shown to other users.
Changed it so that the password is copied locally and the argument cleared.

client/maxadmin.c

@@ -139,6 +139,7 @@ main(int argc, char **argv)
 
             case 'p':
                 passwd = strdup(optarg);
+                memset(optarg, '\0', strlen(optarg));
                 break;
 
             case 'P':