vi(1)-style half page scroll in copy and scroll modes. Move the vi full page

scroll key to C-b instead of C-u and use C-u/C-d for half page scrolling with
vi keys. In emacs mode, half page scrolling is bound to M-Up and M-Down.

Suggested by merdely (about a year ago :-)).

tmux.h

@@ -388,12 +388,14 @@ enum mode_key_cmd {
 	MODEKEYCHOICE_UP,
 
 	/* Copy keys. */
-	MODEKEYCOPY_CANCEL,
 	MODEKEYCOPY_BACKTOINDENTATION,
+	MODEKEYCOPY_CANCEL,
 	MODEKEYCOPY_CLEARSELECTION,
 	MODEKEYCOPY_COPYSELECTION,
 	MODEKEYCOPY_DOWN,
 	MODEKEYCOPY_ENDOFLINE,
+	MODEKEYCOPY_HALFPAGEDOWN,
+	MODEKEYCOPY_HALFPAGEUP,
 	MODEKEYCOPY_LEFT,
 	MODEKEYCOPY_NEXTPAGE,
 	MODEKEYCOPY_NEXTWORD,