Log message:
Add ssh-agent-switcher: SSH agent proxy for tmux

ssh-agent-switcher is a daemon that proxies SSH agent connections to any valid
forwarded agent provided by sshd.  This allows long-lived processes such as
terminal multiplexers like `tmux` or `screen` to access the connection-specific
forwarded agents.

The ssh-agent-switcher daemon solves this problem by exposing an SSH agent
socket at a well-known location, allowing you to set `SSH_AUTH_SOCK` to a path
that does *not* change across different connections.  The daemon then looks for
a valid socket every time it receives a request and forwards the request to the
real forwarded agent.