Fix two copy mode problems:

1. In vi mode the selection doesn't include the last character if you
   moved the cursor up or left.
2. In emacs mode the selection includes the last character if you moved
   the cursor to the left.

From Balazs Kezes.

tmux.h

@@ -794,6 +794,7 @@ LIST_HEAD(joblist, job);
 struct screen_sel {
 	int		 flag;
 	int		 rectflag;
+	int		 modekeys;
 
 	u_int		 sx;
 	u_int		 sy;