Reject ssh changes with invalid root password

[areynold]

Closes #155

To test:

1. Open Administration > Advanced > System > Administration in a web browser
2. SSH into the node and run uci show dropbear
3. Using an incorrect/blank root password, change a number of settings on the page, including ssh keys.
4. Click Save, then run uci show dropbear from the command line. The settings should not have changed. If you added an ssh key, it should not have been added to /etc/dropbear/authorized_keys.
5. Repeat using an incorrect root password and Save & Apply instead of Save. Your settings should not appear in /etc/config/dropbear.
6. Enter the correct root password and repeat. Your new settings should have been saved.