machined: call unlockpt() in container, not host

It makes assumptions about the pty path, hence better call it in the
container namespace rather than the host.

src/basic/util.c

@@ -6095,6 +6095,9 @@ int openpt_in_namespace(pid_t pid, int flags) {
                 if (master < 0)
                         _exit(EXIT_FAILURE);
 
+                if (unlockpt(master) < 0)
+                        _exit(EXIT_FAILURE);
+
                 cmsg = CMSG_FIRSTHDR(&mh);
                 cmsg->cmsg_level = SOL_SOCKET;
                 cmsg->cmsg_type = SCM_RIGHTS;