Bugfixes for retry parameter, large file transfers.

• bugfix: Large file transfers in send() were more likely to fail for
  small values of retry: This value should be the maximum failures per
  block transfer as documented, but was improperly implemented as the number
  of failures allowed for the total duration of the transfer, PR #21 <https://github.com/tehmaze/xmodem/pull/21>_.
• bugfix: send(retry=n) and recv(retry=n) would not retry n-1
  times, rather than n times as documented, causing 'retry=1' to never
  retry, for example.