`$SSH_AUTH_SOCK` must not be set unconditionally through
'home.sessionVariables' since its value needs to be preserved if it
stems from a forwarded SSH agent. Hence, this commit establishes a
centralized, internal 'ssh_auth_sock.initialization' option for setting
`$SSH_AUTH_SOCK`. It checks whether its value has to be preserved and
injects the initialization code into the proper
'programs.(bash|fish|nushell|zsh).*' options.
OpenSSH ssh-agent exits with status 2 when systemd stops it in
non-socket-activated mode.
Home Manager runs ssh-agent that way, so normal user-manager
shutdowns show up as unit failures.
Set SuccessExitStatus=2 for the Linux user service to match
upstream behavior. Startup failures and other unexpected exits
still fail the unit.
