Fix some bugs in blob handling.

- make fd2buf() decrypt the received data.
- stdin_command() has to read from the socket fd rather than from stdin.
  Moreover, it must send the AWAITING_DATA_MSG to the client.
- com_catblob() really needs to use send_bin_buffer() rather than send_buffer().
- com_addblob() has to pass the socket fd to stdin_command().