Port forwarding module now passes backend handles around properly.

As a result I've now been able to turn the global variables `back'
and `backhandle' into module-level statics in the individual front
ends. Now _that's_ progress!

[originally from svn r2142]

window.c

@@ -116,6 +116,8 @@ static time_t last_movement = 0;
 static int caret_x = -1, caret_y = -1;
 
 static void *ldisc;
+static Backend *back;
+static void *backhandle;
 
 #define FONT_NORMAL 0
 #define FONT_BOLD 1