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]

unix/unix.h

@@ -149,4 +149,9 @@ void *sk_getxdmdata(void *sock, int *lenp);
     if (max < fd + 1) max = fd + 1; \
 } while (0)
 
+/*
+ * Exports from winser.c.
+ */
+extern Backend serial_backend;
+
 #endif