Libguile printer for ports on ttys.

[dskr]

The default libguile printer for port objects attempts to print TTY  
names for fds for which are ttys. This printer fails when ttyname  
returns null. This is the case, at least, on Mac OS X for master ptys.

Although Guile cannot typically make a master pty, it can inherit one.  
I am trying to finish an extension library that provides full pty  
support but could use a fix here.

Cheers,
Dan Ridge