add: inform user on missing ssh client or unavailable ssh server

[rustonbsd]

implementing suggestion from #42 for ssh server and client availability. if it is missing we print these messages:

For missing SSH server:

> iroh-ssh server --ssh-port 222`
SSH server not available on port 222, incoming connections will fail. Please ensure you have an SSH server installed and running on port 222.
Error: no ssh server available on specified port

For missing SSH client:

> iroh-ssh user@0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef`
SSH command not found, please make sure your system has an SSH client installed and and that the exact cmd "ssh" is in your PATH