Server-side session limit per client identity

[nicolascbl]

Feature Request

When a client binary is deployed with a persistence mechanism (cron, schtasks..),
each execution creates a new session on the server even if a session for the same host is
already active. This results in session proliferation that operators must clean up manually.

Proposed Solution

Add a server-side option to limit the number of concurrent sessions per client identity
(user.hostname) or per public key:

• --max-sessions-per-identity N in server config
• as a link flag: link --max-sessions 1

When the limit is reached, either:

• Reject the new connection (client retries later)
• Or drop the oldest session in favor of the new one

I actually have a external script polling ls and calling kill -y