serial: making a working version for my CISCO 800 series router

Hi Marc-André,

This patch makes my CISCO 800 series router work nicely over serial.

There is still one issue I am unable to track:
Key presses seem to be sent in pairs, blocked for one second by 'serial->tv.tv_sec = 1'. After I make the router generate lots of output (the command 'show ?' will do), everything starts working as expected: every key I enter is sent to the router instantly.

Any idea what is causing this?