system/inittab: adjust shutdown

Drop null id since that means "don't show anything" for busybox init,
hence the shutdown sequence (/etc/init.d/rcK) doesn't show anything
giving the false impression that it's not being run.
If someone wants a really silent console they'll need to adjust much more than
this anyway.
Also swap the root read-only remount with swapoff since the swap can be
a regular (loop) file in the root filesystem and make the operation fail
resulting in a dirty filesystem.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>