serial ports: is it an upper limit?

There is currently a limit of 26 serial ports in the serial object. I
could look into removing this limit (and coming up with some naming
scheme for ports > 'z') for a future version of the object, but this
is not likely to happen in the next couple of weeks.

I will try to determine the cause of the crash (probably related to
some assumption I've made that no one would ever have more than 26
ports) and get it fixed for the next release, though.

jb