Bugfix issue #7 on crc checksum failure

(Pull PR #8 first for smaller diff)

When a CRC checksum failure occurs and NAK is sent,

1. we should purge any awaiting stdin as suggested by
   ymodem.txt and documented therein, i would suppose
   this may only be for ymodem, which has a kind of
   readahead buffer, though.  For xmodem local testing
   piped with lrzsz, no data is purged.

2. we should read the next start-of-header block, as
   is usually done when the checksum is valid -- this
   causes our 'seq1' to be SOH or STX, and seq2 fails
   1's complement and subsequent failures about unexpected
   sequence occur, otherwise.