Serial back end for Unix. Due to hardware limitations (no Linux box

I own has both an X display and a working serial port) I have been
unable to give this the full testing it deserves; I've managed to
demonstrate the basic functionality of Unix Plink talking to a
serial port, but I haven't been able to test the GTK front end. I
have no reason to think it will fail, but I'll be more comfortable
once somebody has actually tested it.

[originally from svn r6822]