nspawn: allocate the pty used for /dev/console within the container

The console tty is now allocated from within the container so it's not
necessary anymore to allocate it from the host and bind mount the pty slave
into the container. The pty master is sent to the host.

/dev/console is now a symlink pointing to the pty slave.

This might also be less confusing for applications running inside the container
and the overall result looks cleaner (we don't need to apply manually the
passed selinux context, if any, to the allocated pty for instance).