bash_login: add support for showing service log instead of dashboard

This uses an env variable, VALVE_INFRA_FOLLOW_LOG, to control what happens on login. If this parameter is set to 'all', then it shows all logs from journalctl since boot, and continues to follow the logs. If it's set to anything else, it attempts to use the given string as the name of the service to dump logs for and follow. If the var is missing/unset, the dashboard / tmux session is started, which was the default behavior before this patch.