Make sure a serial port exists by determining whether it is possible to drain

Make sure a serial port exists by determining whether it is possible to drain
its RX queue, before we turn on its interrupt or do other things.  If we can't
then the serial port is disabled.

This fixes drain lockups on laptops and newer desktops that occur when a
non-existant serial port is opened and allows us to ship a CD with a getty
on ttyd0 without locking up the system.