util: always enforce O_NOCTTY and O_CLOEXEC in openpt_in_namespace()

The child process is shortliving, hence always set O_NOCTTY so that the
tty doesn't quickly become controlling TTY and then gives it up again.
Also set O_CLOEXEC, because it's cleaner, and doesn't affect the parent
anyway.

src/basic/util.c

@@ -6143,7 +6143,7 @@ int openpt_in_namespace(pid_t pid, int flags) {
                 if (r < 0)
                         _exit(EXIT_FAILURE);
 
-                master = posix_openpt(flags);
+                master = posix_openpt(flags|O_NOCTTY|O_CLOEXEC);
                 if (master < 0)
                         _exit(EXIT_FAILURE);