Add some code bulletproofing, I have had this around for some time

Add some code bulletproofing, I have had this around for some time
I actually thought it was checked in.

This should stop any extra transmit interupts occuring and a possible
crash if another system talks to the tty port before it is completely
configured.

Also allows debugging when no processes have the port open for writing.
If abort is pressed before getty starts.