respond to a port disconnect event by abandoning the transfer
currently in progress.  this is for dialup modems that may
hang up abruptly.

in Kermit.Receive, respond to a bogus request with a NAK, not
an error packet. some PC implementations of Kermit.Send will
not give up if they keep receiving error packets.

be more robust about accepting remote filenames with strange
punctuation.  Ideally, never try to Receive an illegal name.