zmodem_abort_receive(), zmodem_send_ack(), and zmodem_recv_subpacket().
Transfers will now actually work with max_errors set to 0
(attempts=max_errors+1).
zmodem_recv_init() now handles the following headers: ZFILE, ZSINIT, ZCOMMAND,
ZFREECNT, and ZFIN.
bug-fix in zmodem_recv_file_info(): if subpacket is corrupt, send ZNAK.