Refactoring.
- separete routines from xmodem_send_xxx,
  - xmsend_inirial_handshake
  - xmsend_wait_response,
  - xmsend_repeat_eot_and_wait_response
- separete routines from xmodem_receive,
  - xmrecv_drain_pending_chars
- separete routines from xymodem_send,
  - ymodem_send_1k
- rename routines.
  - crc16 --> calculate_crc16
  - update_CRC --> update_crc16
  - xmodem_1k --> xmodem_send_1k
  - xmodem_send --> xmodem_send_128b
  - receive_packet --> xmrecv_receive_packet
- rename struct types.
  - xpacket --> xpacket_128b
- separete xpacket's header / footer
  - xpacket_hdr, xpacket_ftr_crc

Fix error code handling.
- use 'rc' for system call's return code,
  and add 'err' for xymodem function's return code (OK, ERR, ...)
- add ERR_TMO to return codes
- change USER_CAN to ERR_USER_CAN, because it is negative value

Fix xmrecv_receive_packet()'s argument packet to pointer type.

If output-line-delay is non-zero and output-delay is any, line delay
time set output-line-delay.
If output-line-delay is zero and output-delay is non-zero, line delay
time set output-delay.

It was previously (output-line-delay + output-delay) in any case.
Also, since the buffer was flushed after the line delay, there was a
possibility that the delay would be reduced or not applied at all.

When the input mode is set to line, the left bracket cannot be input.

Note: XMODEM_SUM refers to the early XMODEM 128bytes/Checksum.