Fix transport bug and enable RECONNECT by default

(1) No longer check whether transport process is running when reading
    network. This is because even if the transport process has quited,
    the last message is still readable in the output pipe. Transport
    modules gnutls, netcat and openssl are changed due to this.
(2) Added signal handler for SIGTERM.
(3) CBOT_RECONNECT default value is 1. Now the bot will keep
    reconnecting unless an ERROR command is received from server.
(4) Fix OpenSSL issue in modules/command
    OpenSSL disconnects if a 'QUIT' is detected in stdin.
    A lowercase 'quit' does not trigger this behavior.