system/openssh: rename init script to openssh

On shutdown, the initscript will kill any processes that match sshd. However, since the initscript itself is called sshd, it ends up seeing/killing itself! This leads to the shutdown of sshd always thinking there are active ssh connections that need to be shutdown and then failing due to the TERM signal it sent itself.

Rename the script to openssh to avoid this.

NOTE: This change affects any documentation that says:

sudo rc-update add sshd default

It now needs to say:

sudo rc-update add openssh default