Add Jordan Russell's patch for a sunken-edge border like a DOS box.

Configurable, of course, because I for one like the border as thin
as possible.

[originally from svn r1092]

putty.h

@@ -242,6 +242,7 @@ typedef struct {
     int logtype;
     int logxfovr;
     int hide_mouseptr;
+    int sunken_edge;
     char answerback[256];
     /* Colour options */
     int try_palette;