Tailscale SSH: Findings and fixes

Tailscale reported security findings affecting Tailscale SSH and three bugs in Tailscale Serve and Funnel. Tailscale version 1.104.4 contains the fixes. Exploitation of the Tailscale SSH vulnerabilities requires an existing foothold on an authorized device in the tailnet, SSH access explicitly permitted by the tailnet policy file, and the conditions needed to exercise the vulnerable functionality.