namespace: only make the symlink /dev/ptmx if it was already a symlink

…otherwise try to clone it as a device node

On most contemporary distros /dev/ptmx is a device node, and
/dev/pts/ptmx has 000 inaccessible permissions. In those cases
the symlink /dev/ptmx -> /dev/pts/ptmx breaks the pseudo tty support.

In that case we better clone the device node.

OTOH, in nspawn containers (and possibly others), /dev/pts/ptmx has
normal permissions, and /dev/ptmx is a symlink. In that case make the
same symlink.

fixes #7878