Smtp client

    • smtp_forward, moves config loading into exports.register() with a callback
    • line_socket: rename 'self'  -> 'socket', which is more descriptive
    • smtp_client: made timeout handling more robust
        • moved 5 var STATE_* constants into STATE { ... }
        • renamed 'self' -> 'client', which is more descriptive
        • only load tls_key & tls_cert if tls_enabled is enabled