This patch is the first of several planned fixes for the cyclades
multiserial cards driver.

Its mostly a sync with in-house driver:

- Prevent users from opening non-existing Z ports
- Implement special XON/XOFF character handling in Z cards
- Prevent data-loss on Z cards
- Throttling fix for Z card
- Only throttle if CTS/RTS are set
- Fix accounting of received data

Kudos to Cyclades R&D