Don't print the oldest line in the message buffer if the buffer is

Don't print the oldest line in the message buffer if the buffer is
full, since that line is almost always incomplete. Make the parsing
of <%d> lines more strict.

Also simplify the logic a little:
 - Start off by making the buffer linear so that we don't have to
   deal with it wrapping around (suggested by bde).
 - Process line by line rather than byte at a time.