Cleanups from yesterday's destabilisation: lots of stuff in

terminal.c was apparently relying on implicit initialisation to
zero, and also I've removed the backends' dependency on terminal.h
by having terminal sizes explicitly passed in to back->size().

[originally from svn r2117]

putty.h

@@ -194,7 +194,7 @@ struct backend_tag {
     int (*send) (char *buf, int len);
     /* back->sendbuffer() does the same thing but without attempting a send */
     int (*sendbuffer) (void);
-    void (*size) (void);
+    void (*size) (int width, int height);
     void (*special) (Telnet_Special code);
     Socket(*socket) (void);
     int (*exitcode) (void);