[sshd] Limit the number of alive messages.

[sshd] Limit the number of alive messages.

`sshd` keeps idle connection alive for `ClientAliveInterval` seconds
and then sends `ClientAliveCountMax` alive messages. If client does
not respond, `sshd` terminates the connection. This commit limits the
number of alive messages to 1.

Note: `ClientAliveCountMax` applies only to SSH protocol 2.